A functional requirement defines a result, behaviour, or interaction that a system must provide.
It explains what the system must do. For example, a banking system may allow customers to view their account balance or transfer money.
A functional requirement may describe inputs, outputs, calculations, business rules, and user interactions. Moreover, acceptance criteria can define the expected result in measurable terms.
A quality requirement, in contrast, defines how well the system must perform. For example, response time and reliability describe quality. A constraint limits the available solution, such as a required technology or legal rule.
Teams should use clear and testable language. In addition, they should avoid vague terms and unnecessary design decisions.
Clear functional requirements support shared understanding, implementation, verification, and acceptance.
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