When investing in new technology our focus lies not on the products themselves but on the benefits they offer.

Strategies for Effective Functional Requirements And Product Development

When investing in new technology such as cell phones, TVs, or computers, our focus lies not on the products themselves but on the benefits they offer. Similarly, for businesses and governments procuring new systems or enterprise software, the primary concern is how these acquisitions will advance their objectives, enhance efficiency, and contribute to their bottom line or budget utilization.

The essence of a product lies in its functionality—what it can accomplish. Hence, the initial step in product development entails defining its functional requirements. But what exactly are functional requirements, and what role do they play in the development process?

Functional requirements delineate how a product should meet user needs and business objectives. They serve as a blueprint guiding the development team to create a solution that aligns with stakeholders' expectations and delivers tangible value.

Let's delve deeper into functional requirements by examining typical examples and the various types of requirements they encompass. Additionally, we'll offer insights into crafting effective functional requirements and developing comprehensive requirement specifications to steer product development in the right direction.

What are the functional requirements in product development?

Functional requirements outline the specific actions and behaviors

Functional requirements outline the specific actions and behaviors that a product—whether it's a system, subsystem, component, device, or software program—must exhibit. These requirements are essential guidelines for developers to ensure that the product meets user expectations and fulfils its intended purpose.

There are several types of functional requirements, each addressing different aspects of the product's functionality:

  • Operations and Workflows: These requirements define the tasks and processes that the product must execute, detailing its features and functionalities.
  • Data Formats and Validity: Functional requirements also specify the formats and criteria for input and output data, ensuring consistency and accuracy in data handling.
  • User Interface Behavior: They outline how users interact with the product's interface, including navigation, input methods, and visual elements.
  • Data Integrity and Security: Functional requirements pertaining to data integrity and security establish protocols and measures to safeguard sensitive information and maintain data accuracy.
  • Regulatory Compliance: Products must adhere to safety and regulatory standards, and functional requirements articulate how the product meets these requirements.
  • User Access and Authorization: These requirements define how the system verifies and manages user access, ensuring that only authorized users can utilize or modify the system.

Difference between functional requirements and non-functional requirements

Functional requirements are about what a product needs to do, like how it should work.

The term functional requirements implies there must also be a group classified as non-functional requirements. That is indeed the case. Here are definitions of both:

Functional Requirements

Functional requirements are about what a product needs to do, like how it should work. They're like a to-do list for the product's features and actions. These requirements are broken down into smaller tasks as the product is developed and are checked by testing to make sure they work properly. They're always needed for the product to be considered finished.

Nonfunctional Requirements

Non-functional requirements set rules for how a product should be made or how it should behave. They're often based on rules or laws that need to be followed. For example, they might include rules about how a product should be made to work with other products, how it should be supported while it's being used, or how it should be disposed of when it's no longer needed. Unlike functional requirements, they're usually not broken down into smaller tasks. They're checked by looking at the product and its documents to ensure they're being met. Some non-functional requirements are necessary because of rules or laws, while others might be based on goals the company has, like making the product more appealing to customers.

Forms and examples of functional requirements

Functional requirements are essential for defining what a product should do. They come in various forms, each serving specific purposes.

Ubiquitous Requirements

Ubiquitous functional requirements are continuously active, not triggered by events or inputs, and apply universally across all system states.

Template: The shall

Example: The control system shall prevent engine overspeed.

State-driven Requirements

State-driven functional requirements are active as long as a defined state remains true. They are identified with the keyword "WHILE" in methods like Mavin’s EARS.

Template: WHILE the shall

Example: While the aircraft is in flight and the engine is running, the control system shall maintain the engine fuel flow above?? lbs/sec.

Event-driven Requirements

Event-driven functional requirements respond to specific events detected at the system boundary. These events trigger them and are identified with the keyword "WHEN" in methods like EARS.

Template: WHEN the shall

Example: When continuous ignition is commanded by the aircraft, the control system shall switch on continuous ignition.

Optional Feature Requirements

Optional feature functional requirements apply only when specific optional features are present in the system. They are identified by the keyword "WHERE" in methods like EARS.

Template: WHERE the shall

Example: Where the control system includes an overspeed protection function, the control system shall test the availability of the overspeed protection function before aircraft dispatch.

Unwanted Behavior Requirements

Unwanted behavior requirements specify behaviors the system should avoid to ensure correct and safe operation.

Unwanted behavior requirements are essential for scenarios where the system must respond to triggers under less than optimal conditions. These requirements aim to ensure the system behaves correctly even in unfavorable circumstances.

Template: IF , THEN the shall.

Example: If the computed airspeed is unavailable, then the control system shall use modeled airspeed.

Complex Requirements

Complex requirements arise when specific conditions or features must be present for a particular event to trigger a required system response. In such cases, the EARS method combines keywords to express these requirements effectively.

Desired Behavior Template: Where, while , when the shall .

Unwanted Behavior Template: Where, while , if then the shall .

Example: When reverse thrust is commanded while the aircraft is on the ground, the control system shall enable the deployment of the thrust reverser.

Tips for writing good functional requirements

it's considered a best practice to assign a unique identification number or code to each requirement.

Consistently use model verbs

Crafting clear and precise functional requirements is a crucial skill in engineering, one that improves with practice. While an exhaustive list of guidelines is beyond the scope here, six key tips can significantly enhance requirement writing:

Modal verbs like "shall," "must," "will," or "should" convey different levels of necessity, intention, or possibility in engineering specifications. Consistency in their use is vital to avoid ambiguity and ensure clear communication with developers, testers, and regulatory authorities.

Choosing Modal Verbs: "Shall" and "must" are preferred for binding requirements, while "will" signifies expected external behavior. "Should" or "may" can denote non-binding recommendations.

Use of "Must": Some organizations use "must" to express constraints and certain quality or performance requirements, especially in non-functional requirements. This clarifies distinctions between functional ("shall") and non-functional requirements.

Clarification: Define the interpretation of modal verbs within the document, especially in non-requirements documents referenced. This is often addressed in a dedicated section at the beginning of the specification.

These practices ensure consistent understanding and interpretation of requirements, promoting clarity and effectiveness in system development.

Tag each requirement with a unique identifier

In software and systems engineering, it's considered a best practice to assign a unique identification number or code to each requirement. This practice is often a requirement itself, particularly in contracts between customers and suppliers, where adherence to industry standards like IEEE/EIA 12207 is mandated.

Assigning unique identifiers to requirements offers significant benefits to system developers:

  • Facilitates Traceability: Tagging each requirement with a Project Unique Identifier (PUI) simplifies traceability between requirements across different design levels and the corresponding verification tests. This traceability is crucial for ensuring that each requirement is adequately tested and verified.
  • Simplifies Documentation: Brief identifiers enable the creation of traceability tables that clearly link each requirement to its parent in higher-level documents and to the specific tests designed to verify it. This documentation simplifies the process of demonstrating compliance with top-level requirements to both customers and internal stakeholders.
  • Streamlined Process: Automated requirements management tools often include features for automatically assigning unique identifiers. This automation streamlines the process, reducing the risk of errors and ensuring consistency across requirements documents.

Write only one requirement in each requirement statement

Complex requirement statements pose a potential pitfall in requirement engineering, especially those containing multiple modal verbs or the word "and." Such statements can often encompass multiple requirements within a single paragraph or sentence.

Long, convoluted requirement statements can obscure clarity and hinder effective communication. Here's what to watch out for:

  • Identify Complexity: Statements with "and" or multiple modal verbs likely contain more than one requirement. It's essential to recognize such complexity.
  • Analyze and Simplify: Take the time to dissect lengthy requirement statements. Rewrite them into simpler, individual requirements, each with its own modal verb like "shall."
  • Assign Unique Identifiers: Once broken down, assign a unique identifier to each simplified requirement. This ensures clear traceability and documentation.

Simplifying complex requirement statements into clear, concise units enhances readability, understanding, and manageability throughout the development process. By avoiding overly complicated requirements, teams can streamline communication, mitigate risks of misinterpretation, and promote more efficient system development.

Write requirements statements as concisely as possible

Analyzing and rewriting long requirements, even those containing a single "shall," is crucial because lengthy statements are more prone to misinterpretation compared to short, concise requirements.

Writing requirements that are as brief as possible is a fundamental practice in software and systems engineering. Here's why:

  • Reduced Misinterpretation: Long requirements increase the risk of ambiguity and misunderstanding. Concise requirements leave less room for interpretation errors, ensuring clarity and alignment among stakeholders.
  • Enhanced Readability: Short, clear requirements are easier to read and understand for everyone involved in the development process. They promote effective communication and minimize confusion.
  • Use of Templates: Requirements templates, such as the EARS patterns discussed earlier, offer valuable guidance in crafting concise requirements. These templates provide a structured approach to requirement writing, helping to maintain clarity and brevity.

Make sure each requirement is testable

Before finalizing a new functional requirement, it's essential to consider how its successful implementation will be verified. This question serves as a guiding principle in requirement writing, ensuring clarity and alignment between design and test engineers.

Here's why verifying requirements with specific test scenarios in mind is crucial:

  • Clarity and Understanding: Specifying requirements with test scenarios helps ensure that both design and test engineers fully grasp what needs to be accomplished. This clarity minimizes misunderstandings and promotes effective collaboration.
  • Influence on Detail: The nature of the test case influences the level of detail required in the requirement. High-level requirements, often tested through inspection or user testing, may have broader scopes. In contrast, lower-level requirements verified through software testing or system integration testing necessitate more detailed specifications.

Clearly segregate requirement statements from rationale and other explanations

In requirement specifications, it's crucial to provide contextual information alongside the requirement to enhance clarity and understanding for developers and testers. Context helps prevent misinterpretation by eliminating ambiguities and allows stakeholders to fully grasp the requirement's intent, facilitating valuable feedback for refinement. However, it's essential to segregate contextual information from the requirement statement itself to maintain clarity and avoid dependency on implementation or testing considerations. A best practice is to include contextual information in a separate paragraph without a unique identifier, ensuring the requirement remains concise and independent. Utilizing a Functional Requirements Document template or Requirements Management tool can streamline this process, facilitating effective communication and collaboration among project stakeholders.

What is a Functional Requirements Document?

A Functional Requirements Document (FRD), also known as a Functional Requirements Specification or Functional Specification Document, serves as a detailed counterpart to the higher-level Product Requirements Document (PRD). While the PRD outlines broader product needs, the FRD delves into technical specifics after analyzing and decomposing the PRD requirements.

The FRD articulates what the system user requires, primarily focusing on system outputs in response to inputs. It provides precise functional requirements and guidance for developers and testers, detailing interactions with external agents such as users, peripherals, or other systems.

Notably, the FRD does not dictate the system's internal workings or implementation methods; rather, it communicates observable behaviors to stakeholders. Developers learn what to build, testers understand testing requirements, and stakeholders receive a comprehensive description of the product.

For complex systems, functional requirements may undergo further decomposition into various specifications, including System, Subsystem, System Component, and Software Requirements Specifications. Additionally, in some industries, non-functional requirements may be integrated into larger specifications alongside functional requirements, although typically segregated for clarity.

Tips for compiling a good FRD

Compiling an effective Functional Requirements Document (FRD) requires careful organization and structure.

Compiling an effective Functional Requirements Document (FRD) requires careful organization and structure. Here are some tips to ensure your FRD is cohesive, usable, and easily navigable:

Hierarchical Structure

Organize the document hierarchically to enhance comprehension and usability. Consider structuring it based on mission/phase/function, function/subfunction, or feature/use case. This hierarchical approach offers several advantages:

It allows contributors to focus on specific domains, facilitating targeted discussions and analysis.

Contributors can easily locate and modify relevant sections when adding functionality to an existing system.

Users, such as developers and testers, can quickly navigate to the specific functional areas they need to reference or work on.

Standardize your requirements language

Standardizing your requirements language is essential for clarity and precision in communication. Spoken languages often harbor words with multiple meanings, leading to ambiguity and potential misinterpretation. By establishing a glossary of standardized terms at the outset of your requirements document, you can mitigate confusion and foster shared understanding.

This glossary serves as a reference point, delineating precisely how specific terms are to be utilized within the document and how they should be construed when encountered in supplementary materials. By rigorously defining terms and adhering strictly to these definitions, you not only streamline comprehension among stakeholders but also optimize efficiency in requirement drafting.

Embracing standardized language enhances clarity and cultivates a more efficient workflow. Over time, as familiarity with these terms grows, this approach reduces ambiguity and accelerates requirement formulation.

Use a good FRD template or a dedicated RM platform

When embarking on the creation of a functional requirements document (FRD), it's akin to laying the groundwork for a structure; starting with a reliable template is paramount. A well-designed template encompasses standardized sections that serve as the backbone for consistency and clarity throughout the document.

These standardized sections typically include guidelines for the use of modal verbs, a glossary of standardized terms, instructions for documenting requirements, guidelines for managing requirements documentation, and any other organizational protocols. Referred to as "boilerplate," these sections promote uniformity across projects and teams, fostering cohesion and efficiency.

One of the key advantages of utilizing templates is their stability across projects and teams. While they may undergo gradual evolution in response to methodological shifts and accrued knowledge, their core components remain largely unchanged. This stability provides a reliable framework for consistent requirements development, employee training, and effective communication with stakeholders.

Numerous FRD templates are readily available online, offering a starting point from which to tailor documentation to specific project needs while maintaining adherence to best practices.

The drawbacks of templates

While using general documentation platforms like Word, Excel, or Google Docs for documenting requirements may seem convenient, it comes with its own set of limitations. Collaboration often becomes cumbersome, and these platforms lack the robust features necessary to support clear and systematic requirements traceability.

According to Gartner, relying on such general document software hampers companies' ability to achieve adequate visibility and traceability in their requirement specifications. This reliance leads to poorly managed requirements, scattered across various documents and spreadsheets, often supplemented by disjointed post-it notes. The result? A costly user acceptance testing cycle, prolonged execution times, and increased remediation efforts for issues discovered late in the process.

To ensure product success, it's imperative to employ a robust requirements management (RM) platform. Such platforms offer fields, formatting options, and structural relationships that facilitate portability of boilerplate sections across projects, precise requirements definition and identification, and comprehensive traceability to higher-level and lower-level requirements as well as test cases.

A top-tier RM platform goes beyond basic functionalities by enabling seamless team collaboration. By providing all users with access to the latest requirements baseline and pending changes, these platforms streamline requirements tracking, traceability, and test coverage assurance. RMsis is a prime example of a solution that excels in these areas, offering unparalleled efficiency in tracking and tracing requirements for successful project outcomes.


In conclusion, developing functional requirements documents (FRDs) is a pivotal step in creating any product, system, or software. These documents serve as blueprints, guiding development teams towards building solutions that meet user needs and align with business objectives. By delineating specific actions, behaviors, and functionalities, functional requirements ensure clarity, consistency, and traceability throughout the development process.

While general documentation platforms may offer convenience, they often lack the robust features necessary to support clear and systematic requirements traceability. Investing in a dedicated requirements management platform, such as RMsis, can significantly enhance visibility, traceability, and collaboration, ultimately contributing to the success of the product or project.

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