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Non-Functional Requirements
Non-Functional Requirements specify how the system should do it. Non-functional requirements do not affect the basic functionality of the system (hence the name, non-functional requirements). Even if the non-functional requirements are not met, the system will still perform its basic purpose.
If a system will still perform without meeting the non-functional requirements, why are they important? The answer is usability. Non-functional requirements define system behavior, features, and general characteristics that affect the user experience. How well non-functional requirements are defined and executed determines how easy the system is to use, and is used to judge system performance. Non-functional requirements are product properties and focus on user expectations.
See also Functional Requirements.
Source: Non-Functional Requirements
