A service-level agreement (SLA) relates to a particular element, aspect or range of activities where a service is formally defined in a written form.

In particular the different aspects of the service or relationship - scope, quality, responsibilities - are agreed between the service provider and the service user. Service Level Agreements may be between two or more parties where one is the customer and the others are service providers.

The purpose is to support the most efficient working practices and to generate savings in the business process.
Note that - SLA’s are not normally bound by legal agreement (means contracts) such contracts between the service provider and other third parties may in fact sometimes be (incorrectly) called SLAs - Why? - Because the level of service has been set by the (principal) customer, (means it is a based not on a SLA but rather on a business requirement) - so it is an instruction!