A functional requirement describes what the software must do, including behavior, processing rules, inputs, outputs, calculations, validations, and responses to events. The statement “The system must reject non-numeric input” defines a specific behavior the system must perform when invalid input is entered, so it is a functional requirement. The 99.9% operational requirement describes availability, which is a non-functional quality attribute. The response time requirement describes performance, another non-functional requirement. The requirement to develop according to IEEE standards describes a process, compliance, or constraint requirement rather than system behavior. Software quality engineers distinguish these requirement types because they affect design, verification, validation, traceability, and test planning differently.

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