What is an example of a non-functional requirement?

Non-functional requirements define how a system performs a particular function, rather than what the system does. They are critical for determining the quality attributes of a system, such as performance, usability, reliability, and security.

In the context of the chosen example, performance metrics for response time fall squarely within the realm of non-functional requirements because they specify how quickly a system should respond to user actions or system events. This defines a quality standard that the system must meet, which is fundamental in ensuring a satisfactory user experience.

The other examples presented relate more to functional requirements. For instance, user authentication procedures, user roles and permissions, and data storage format all describe specific functionalities or features that the system must provide. They focus on "what" the system should do rather than "how" well it should perform under different circumstances. Thus, focusing on performance metrics distinctly highlights the non-functional aspect of the requirement.