Explanation/Reference:
The three primary parts of a SOA Service as defined by ORA are contract, interface, and implementation.
Note:
A Service Contract describes the SOA Service in human-readable terms.
The Service Implementation is the technical realization of the contract. I A Service Interface provides a means for the consumers of a SOA Service to access its functionality according to the Service Contract.
Reference: Oracle Reference Architecture, Service-Oriented Integration, Release 3.0