Functional Requirements
Functions are discrete, testable system behaviors. The atomic unit of requirements. One input, one expected output. Answer: How exactly must it behave?
UID prefix: FN- Traces up to: Features (FEAT-) Implemented by: Source code (# impl FN-NNN) Verified by: Tests (# verify FN-NNN)
The system SHALL [specific measurable behavior].
Input: [what triggers this] Output: [what the system produces] Constraints: [timing, format, limits] Verification method: [test / inspection / analysis / demonstration]