Glossary · Requirements and interfaces
Nonfunctional requirement
Also known as: non-functional requirement, NFR, Quality requirement
German: Nichtfunktionale Anforderung
In systems engineering, a nonfunctional requirement is a requirement defining qualities or constraints such as timing, safety, security, usability, availability, maintainability or resilience. It describes how well or under which conditions the system performs its functions.
- Systems engineering
- Verification
In one sentence
A nonfunctional requirement defines qualities like timing, safety, security or maintainability, which often decide whether a function is safe in practice.
Example
“The robot shall reach standstill within 300 ms after the guard is opened” adds a nonfunctional timing requirement to the functional requirement to stop.
How it applies
- Functional safety: Integrity targets (PL or SIL), response times and diagnostic requirements are nonfunctional parts of a safety function specification.
- Verification: Nonfunctional requirements need measurable criteria. “Fast”, “robust” or “user-friendly” are not verifiable without a verification criterion.
- Integrated systems: Timing and availability requirements often span several subsystems. Allocate budgets, such as latency, to each part.
- Technical documentation: Usability and readability of safety information are nonfunctional requirements for the documentation itself.
Nonfunctional vs. functional requirement
A functional requirement states what the system does. A nonfunctional requirement states how well or under which constraints. Many safety requirements combine both.