Some Spectrum OnDemand modules come with standard web services, such as the ValidateAddress web service that comes with the Spectrum Universal Address. These web services support GET only. You can add POST support to these standard web services by creating a user-defined service in Spectrum Enterprise Designer and placing the standard service in it as a stage. Since you can expose user-defined services with POST support, you are in effect creating a web service that exposes the standard service with POST support.
You have created a user-defined web service that exposes a standard service as a REST web service that supports POST.