SLA based services are services that are based on a service level agreement (SLA). This is a contract between a service provider and the end user that sets out the level of service that the service provider is expected to offer.
They define what the customer is going to receive in exchange for their money, although not how the service will be delivered. An SLA should include a number of different things. It might feature a description of the service, when the service will be available and how quickly it will be delivered when it’s needed.
More info @ metrics to consider while choosing SLA