Requirement statement	A statement of need with which compliance must be demonstrated; this implies an acceptance test and an acceptance authority. Often expressed as an entry in a requirements catalogue or a use case description. Requirements attributes are likely to include: reference number, description, source, owner, type, priority, deadline. A requirement should be SMART, which implies the definition of acceptance tests.
Functional requirement	A requirement related to data into or output from a system, and to processes and business rules for input and output.
Audit requirement	A requirement to do with ensuring an auditor can find the when/where/how/who of a process or stored data, and can replay events. (Considered by some to be a functional requirement and others to be non-functional.)
Non-functional requirement	A requirement about the ability of a system to perform its functions (whatever they are) effectively and efficiently. Usually quantitatively measurable.
Performance	Subdivides into two measures, often in opposition: · Throughput: number of services executed in a time period. · Response or cycle time (aka latency): time taken from request to response.
Availability	The amount or percentage of time that the services of a system are ready for use, excluding planned down time.
Recoverability	The ability of a system to be restored to live operations after a failure.
Reliability	The mean time between failures. Usually applied to the technologies in the Infrastructure, ignoring the more likely risk of application failure.
Integrity	See Data integrity and Data flow integrity.
Scalability	The ability of a system to grow to accommodate increased work loads.
Security	The ability of a system to prevent unauthorized access to its contents.
Serviceability	The ability of operations team to monitor and manage a system in operation.
Usability	The ability of actors to use a system.
Maintainability	The ability of maintenance teams to revise or enhance a system.
Portability	The ability to move a component from one platform to another, or convert it to run on another platform. In practice, it can be difficult to set or estimate this quality metric realistically.
Interoperability	The ability for subsystems to exchange data using shared protocols and networks.
Integratability	The ability of interoperable subsystems to understand each other, which requires either common data types or brokers to translate between data types.
Service Level Agreement (SLA)	“Written agreement between an IT service provider and customer (s) that documents agreed-to service levels.” (ITIL) A document defining the requirements that a service (definition 2) must meet, often focused on non-functional requirements.
Service Level Requirement (SLR)	“Criteria for level of service required to meet business objectives.” ITIL