Which term refers to requirements that constrain how a system behaves?

Non-functional requirements refer to the criteria that define the operation of a system rather than specific behaviors or functions it must perform. They describe the qualities or attributes that the system should exhibit, such as performance, usability, reliability, security, and scalability. These requirements set constraints on how a system behaves but do not dictate what the system should do; rather, they define how well it should perform those tasks.

For example, a non-functional requirement might specify that a website must load within two seconds, ensuring a good user experience, or that it must handle a certain number of simultaneous users, guaranteeing reliability under load. These aspects are crucial for ensuring that user satisfaction is met and that technical and business requirements align effectively.

In contrast, functional requirements describe specific functionalities or features that a system must have, such as the ability to log in or retrieve user data. System specifications encompass the detailed aspects and features of a system but may not categorize requirements into functional or non-functional. User feedback is input from users regarding their experience and satisfaction with the system, which can influence both functional and non-functional requirements but does not define them directly.