The following people use the service interface:

• Implementers of the services, for an understanding of the interface the service provides, but also the behavior its clients expect.
• Implementers of service clients, for an understanding of the interface the service provides, but also how the service expects to be interacted with.
• Designers of services, in understanding the relationship between specifications and the relationship between services and the specifications they implement.
• Those who design the next version of the system, to understand the functionality in the service model.
• Those who test the classes, to plan testing tasks.

The service specification has to provide both the provider (implementer) of a service and the consumer of a service with a reasonable and complete specification of the following aspects:

• Interface Specification; this specifies the set of operations provided by a service realizing this specification. Each operation is named and provides a signature composed of input, output, and exception messages.
• Behavioral Specification; this specifies the protocol between the service and the consumer. A service may be stateful (either explicit or implicit - see the guideline State Management for Services) or it may have certain conversational requirements fulfilled by the client.
• Policy Specification; this specifies constraints and policies regarding the operation of the service. Examples of policies include security, availability, quality of service and so on; these also represent non-functional requirements of the solution as a whole.