Functional requirement
A requirement about behavior or features: the user-visible flows the system must support.
Definition
A functional requirement describes what the system must do.
It should be expressible as an end-to-end flow and validated with acceptance criteria.
How to write one
Start with a user and an outcome:
“A user does X, the system does Y, and the outcome is Z.”
Then list the minimum set of flows that make the system real.
Why it matters
Functional requirements give your diagram meaning. Without flows, boxes are just vibes.