Glossary · Requirements and interfaces
Functional requirement
German: Funktionale Anforderung
In systems engineering, a functional requirement is a requirement defining what the system must do. It describes a function, a transformation or a reaction to an input or event.
- Systems engineering
- Verification
In one sentence
A functional requirement says what a system must do; how well it does it is left to nonfunctional requirements.
Example
“When the interlocking guard of the robot cell is opened, the robot shall initiate a stop” is a functional requirement for the guard interlock.
How it applies
- Requirements: Write functional requirements as observable behavior with a clear trigger and response. Pair each with a verification criterion.
- Functional safety: A safety function is specified by a functional part (what it does) and by integrity and timing requirements. The functional part alone is not enough.
- Technical documentation: Functional requirements are a good source for operating instructions: what happens when the user does X. Keep the trace so instructions follow requirement changes.
- Evidence: Functional tests verify these requirements; nonfunctional properties need their own tests.
Functional vs. nonfunctional requirement
A functional requirement says what the system does. A nonfunctional requirement says how well, how fast, how safely or under which constraints it does it. Safety requirements usually have both parts.