What is the relationship between functional and non-functional requirements?

The relationship between functional and non-functional requirements is well defined within the context of systems and software development. Non-functional requirements indeed describe how functional requirements perform.

Functional requirements define what a system should do, outlining specific behaviors or functions, such as transactions the system must handle or data it must process. In contrast, non-functional requirements focus on the quality attributes of these functions, including performance, usability, reliability, and security. They establish the criteria that can be used to judge the operation of a system rather than the specific behavior.

For example, while a functional requirement might state that the system must allow users to log in, a non-functional requirement would specify that the login process should be completed within two seconds. This distinction emphasizes that non-functional requirements play a crucial role in shaping user experience and satisfaction, affecting how well the functional requirements actually serve the user's needs. Thus, understanding this relationship is essential for developing systems that meet both user expectations and quality standards.