What do non-functional requirements describe?

Non-functional requirements are crucial aspects of system specifications that focus on "how" the system behaves rather than what it does. These requirements address various quality attributes that the system must satisfy, which can include performance, usability, reliability, and security, among others. For instance, a non-functional requirement might specify that a system should respond to user inputs within two seconds, indicating a performance expectation.

This distinction is essential because it highlights that non-functional requirements complement functional requirements, which detail specific behaviors or functions of the system. While functional requirements answer the question of "what" the system should do, non-functional requirements influence the user experience and system performance, shaping how the system will be perceived and used in real-world scenarios.

The other options either misinterpret or dismiss the importance of non-functional requirements. They do not accurately capture the essence of what these requirements entail or their significance in system development.