Non-functional requirements are also referred to as quality attributes or quality-of-service requirements. 

Non-functional requirements are most effective when they are expressed in textual descriptions and include measurable qualities. Measurements can be expressed as percentages or numerical values or in time (such as seconds or minutes).

The most common non-functional requirements categories include availability, compatibility, functionality, maintainability, performance efficiency, portability, reliability, scalability, security, usability, certification, compliance, localization, service-level agreements, and extensibility.