What best describes non-functional requirements?

Non-functional requirements are crucial to understanding how a system operates beyond its core functionalities. They indeed describe "how" the system should work, focusing on the quality and performance aspects that contribute to the overall user experience. These include attributes like reliability, performance, security, usability, maintainability, and scalability.

By emphasizing these aspects, non-functional requirements help stakeholders understand the expectations and constraints that the system must adhere to, ensuring that it not only performs tasks but does so in a manner that meets identified criteria for quality. This holistic view is essential for successful system design and implementation.

The other responses do not accurately capture the essence of non-functional requirements. For instance, describing non-functional requirements as actions the system could take that would make it cease to function misrepresents their nature; instead, these requirements focus on quality attributes rather than failure states. Similarly, while non-functional requirements do encompass quality of service (QOS) elements, reducing them solely to that definition restricts the broader scope they entail. Finally, claiming that non-functional requirements do not exist contradicts established understanding and categorization in systems and software engineering.