Explanation/Reference:
The Service Contract defines what the SOA Service agrees to provide to the environment.
The service consumer Usage Agreement defines what a particular service consumer is entitled to consume.
Each service might have several consumers.
The Service provider must ensure that the Service will satisfy the aggregate specifications of all related usage agreements.
Note:
The usage agreement is not part of the Service; rather it defines what a particular service consumer is entitled to consume from the Service.
Having both a usage agreement and a service contract provides a decoupling between the service provider and service consumer. This not only facilitates reuse but also provides a separation of concerns.
The service contract defines the totality of what the Service guarantees to provide, and can be written and validated independent of any knowledge of specific service consumers. The usage agreement is service consumer specific and defines what capabilities of the Service each consumer is allowed to consume.
Reference: Oracle Reference Architecture and Service Orientation, Release 3.0