SDL contracts define the capabilities, schemas and locations of services, but do not themselves enable developers to directly program against them. They do provide the wire-format details necessary for a developer to use a network level API, like the Net Classes, to directly program against them. But there is an easier way to use these services: by creating client proxies.
ASP+ includes a command-line utility, WebServiceUtil.exe, that automatically creates strongly-typed, NGWS client proxy classes from SDL contracts. Each service method is translated into a class method. These proxy classes automatically abstract all of the network and marshalling protocols necessary for the end user and enables programmers to easily invoke and leverage a web service’s functionality much like they would a regular class or business component.