Functional requirements expressed in user stories, features, and capabilities represent most of the work in building solutions that deliver value to the user. Although they may be a bit subtler, NFRs are just as important to system success as they describe critical operational qualities required for release (or manufacture, or sell).
NFRs are not themselves backlog items. They are constraints on development that limit some degree of design freedom for those building the system. These constraints are often defined in the acceptance criteria for multiple backlog items. For example, SAML-based Single Sign-on (SSO) is a requirement for all products in the suite. SSO is a functional requirement, while SAML is a constraint. And any backlog item building sign-on functionality would reference the SAML constraint in its acceptance criteria. ?FURPS? is a commonly referenced set of important quality attributes: Functionality, Usability, Reliability, Performance, and Supportability [1]. The NFR list is more exhaustive and includes compliance, security, resilience, privacy, accessibility, and others