Services are described by schema and contract, not type

Connected Services Framework
Unlike previous types of systems, the Web-service model does not operate on the notion of shared types that require a common implementation. Rather, services interact based solely on contracts (WSDL/BPEL4WS for message-processing behavior) and schemas (WSDL/XSD for message structure). This enables the service to describe the structure of the messages it can send and/or receive and the sequencing constraints on these messages. The separation between structure and behavior and the explicit, computer-verifiable description of these characteristics simplifies integration in heterogeneous environments.

Furthermore, this information sufficiently characterizes the service interface so that application integration does not require a shared execution environment to create the messages' structure or behavior.

The service-oriented model assumes a fully distributed environment, where it is difficult, if not impossible, to propagate changes in schemas and/or contracts to all the parties that have encountered a service. Service orientation implies that contracts and schemas should remain backward compatible and that they might contain information that is incompletely understood by particular processing systems.

For that reason, the contract and schema technologies designed for use in service-oriented designs enable more flexibility than traditional object-oriented interfaces. In particular, services use features such as XML-element wildcards (for example, xsd:any), schema extensions, and optional SOAP header blocks to evolve services in ways that do not break deployed applications. These characteristics are the key to the composability of Web services.