Functional Requirements

Functional requirements define the specific behaviors, functions, and capabilities that a system or component must exhibit. They answer the question 'what must this do?' — as opposed to non-functional requirements, which answer 'how well must it do it?' Functional requirements describe inputs, outputs, and the transformations between them. They are testable: a good functional requirement can be verified with a pass/fail test.

Well-written functional requirements follow the imperative form: 'The system shall [action] [object] [under what conditions].' For hardware: 'The motor controller shall maintain target speed within ±2 RPM under loads from 0 to maximum rated torque.' The word 'shall' denotes a mandatory requirement; 'should' denotes a desired but optional capability. IEEE 29148 recommends avoiding vague qualifiers like 'easy to use,' 'fast,' or 'reliable' in functional requirements — these are non-functional concerns that require quantified metrics.

Functional requirements drive concept selection and component specification. For each functional requirement, engineers identify candidate implementations (concepts), evaluate trade-offs, and select one. The selected concept's BOM items directly implement the requirement. This link — requirement → concept → component — forms the backbone of product traceability and is essential for managing engineering changes without unintended scope creep.