A service level agreement (SLA) specifies minimum performance requirements and, upon failure to meet those requirements, the level and extent of customer support that must be provided. Service level requirements are system requirements that specify the conditions upon which the SLA is based.
As with QoS requirements, service level requirements derive from business requirements and represent a guarantee about the overall system quality that the deployed system must meet. Because the service level agreement is considered to be a contract, specification of service level requirements should be unambiguous. The service level requirements define exactly under what conditions the requirements are tested and precisely what constitutes failure to meet the requirements.