A proxy service provides a well-defined SOAP endpoint to the outside. In most of the cases, a proxy service, as its name implies, proxies a real, concrete business service. A proxy service may or may not have a one to one mapping to a business service. It can simply provide a level abstraction over one concrete service or many other business services. In WSO2 ESB, a proxy service is built with an In-Sequence, an Out-Sequence, and a fault sequence.
A request message coming in to a given proxy service will hit the In-Sequence defined for that proxy service.
A response message coming from a concrete or a business service will go through the Out-Sequence defined for the corresponding proxy service.