A functional requirement specifies what an application must do so users can complete their tasks. It describes required functionality rather than general qualities such as performance, security, or availability. section 1.2.1
Spacecraft control
A spacecraft application has a functional requirement to let an astronaut control the ship’s systems manually and automatically. Robustness, fault tolerance, security, and compatibility are nonfunctional requirements instead. section 1.2.1
Functional requirements guide the transition from gathered requirements into software design. They clarify the application’s intended behavior while leaving qualities such as maintainability and stability to separate requirements. section 1.2.1
