The devil is in the details.
Nonfunctional Requirements (NFRs) define system qualities such as security, reliability, performance, maintainability, scalability, and usability. They serve as constraints or restrictions on the design of the system across the different backlogs.
In contrast, functional requirements are largely expressed in user stories, features, and capabilities. This is where most of the work occurs. Teams build systems that deliver value to the user, and most of the time and effort in solution development are devoted to that goal.
NFRs, however, ensure the usability and effectiveness ...