How To Write User Stories For Non Functional Requirements . If a functional specification “describes how a product will work entirely. “a type of requirement that describes the performance or quality attributes a solution must meet. NonFunctional Requirements (NFR) in Agile Practices from www.c-sharpcorner.com They ensure the usability and effectiveness of the entire system. “a type of requirement that describes the performance or quality attributes a solution must meet. User stories are written using the following formula: