What is the difference between functional and non-functional requirements?
Functional requirements describe what the system must do (specific behaviors and features); non-functional requirements describe how well it must do it (performance, security, availability, usability).
Full Answer
A functional requirement is a testable statement of system behavior — for example, "the system shall allow a user to reset their password via email." A non-functional requirement constrains the quality of that behavior — for example, "password reset emails must be delivered within 30 seconds for 99% of requests."
Non-functional requirements are the most commonly under-documented category in BRDs, because they are not obvious from a feature list — they must be explicitly elicited from IT, security, and compliance stakeholders, not assumed.
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