Requirements Development

Requirement Types

A requirement is a statement that identifies a product or process operational, functional, or design characteristic or constraint. Users, System Engineers, and Program Managers will have to develop several different types of requirements for an acquisition program through its life cycle. These requirements range from very high-level concept-focused to very specific for a part. The four (4) main types of requirements that you can expect on a program are:

  • Functional Requirements
  • Performance Requirements
  • System Technical Requirements
  • Specifications

Definition: A requirement is a statement that identifies a product or process operational, functional, or design characteristic or constraint, which is unambiguous, testable or measurable, and necessary for product or process acceptability. (ISO/IEC 2007)

Functional Requirements

A functional requirement is a task (sometimes called action or activity) that must be accomplished to provide an operational capability (or satisfy an operational requirement). Some functional requirements associated with operations and support can be discerned from the needed operational capability (see Operational Requirements). Others often result only from diligent systems engineering.

Functional requirements define what a product must do and its features, functions, and specific capabilities.

Experience in systems engineering has identified eight generic functions that most systems must complete over their life cycle: development, manufacturing, verification, deployment, training, operations, support, and disposal. These are known as the eight primary system functions. Each must usually be considered in identifying a system’s functional requirements.

Performance Requirements

Performance requirements define the standards that determine a system’s effectiveness, responsiveness, and scalability. They are frequently classified as non-functional requirements. Performance requirements concentrate on how well a system operates under different circumstances, as opposed to functional requirements, which list specific capabilities. This encompasses variables like resource usage, throughput, and response times. Performance criteria and other non-functional requirements are essential to guarantee that a system fulfills user expectations and performs at its best. They direct implementation and design, highlighting elements crucial to system stability and user experience.

Performance requirements define how the system should perform.

A performance requirement is generally measured in quantity, accuracy, coverage, timeliness, or readiness. The performance requirements for the operational function and sometimes a few others often correlate well with the statement of the needed operational capability developed by the Joint Capabilities Integration and Development System (JCIDS) Process. The statement of other performance requirements usually requires thorough systems engineering.

System Technical Requirements

System technical requirements refer to the specific hardware, software, and network configurations necessary for a particular system or software application to function optimally.  System technical requirements are the result of both allocated and derived requirements. These types or requirements are:

  • Allocated Requirements: flow directly from the system requirements down to the elements of the system.
  • Derived Requirements: These are dependent on the design solution (and so are sometimes called design requirements). They include internal interface constraints between the elements of the system.


A specification is a detailed, exact statement of particulars, especially a statement prescribing materials, dimensions, and quality of work for something to be built, installed, or manufactured. The overall purpose of a specification is to provide a basis for obtaining a product or service that will satisfy a particular need at an economical cost and to invite the maximum reasonable competition. By definition, a specification sets limits, eliminating or potentially eliminating items outside the boundaries drawn. A good specification should do four (4) things:

  1. Identify minimum requirements.
  2. List reproducible test methods to be used in testing for compliance with specifications.
  3. Allow for a competitive bid.
  4. Provide for an equitable award at the lowest possible cost.

Key Takeaways for Understanding Requirement Types

When figuring out the right types of requirements for a project or program, there are a few essential things to remember:

  • Knowing the difference between “needs” and “wants.” Needs are requirements necessary for the project or program to work, while wants are things that would be nice to have but are not always necessary.
  • Different kinds of requirements meet different kinds of needs. For example, functional requirements describe what the system should do, while non-functional requirements describe how the system should act.
  • Specific, measurable, achievable, relevant, and time-bound requirements are needed (SMART). This ensures they are clear and well-defined, making the project or program easier to run.
  • Write Them Down: It is important to figure out and write down all of a project or program’s functional and non-functional requirements. This will help ensure the project or program is finished on time and on budget.
  • Managed and Tracked: During the whole project or program, requirements should be managed and tracked to ensure that they are being met and that the project or program stays on track.

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Other Types of Requirements

Not all requirements are as described above; there are additional kinds as well. These help shape the four primary requirement categories, even though they are not the primary requirements often associated with the actual development of a program.

  • Business Requirements: articulate an organization’s goals and intended results. Engineers who don’t understand how they will be implemented frequently dismiss them as “fluffy,” but if they are stated clearly, they can be reduced to quantifiable claims. Usually, the product owner or sponsor, the marketing division, or the client defines them in a business case or other documents. They try to explain the organization’s financial and resource commitment to the project.
  • Stakeholder Requirements:  outline the characteristics that must be satisfied in order for the business requirements to be satisfied. They also express the demands and expectations of the stakeholders. While performance and reliability as well as a range of other non-functional needs, may be expected by stakeholders, analysts often concentrate on the functional components of the needs. Both are essential and serve as stepping stones for defining the functional and non-functional requirements that the designers and implementers must achieve to produce solutions that meet the client’s expectations.

Requirement Standards

Requirement standards play a crucial role across industries by providing a consistent framework for capturing, documenting, and managing project requirements. These standards ensure clarity, completeness, and traceability of requirements, enhancing communication between stakeholders and minimizing misunderstandings. Industries such as aerospace, automotive, healthcare, and software development rely on specific requirement standards like DO-178C, ISO 26262, IEC 62304, and IEEE 830. These standards define best practices for requirements engineering, including requirements elicitation, analysis, verification, and validation. Compliance with these standards improves product quality, reliability, and safety while reducing risks and development costs. Adhering to requirement standards promotes interoperability, collaboration, and consistency across industries, fostering successful project outcomes.

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Updated: 2/8/2024

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