It is worth noting again that although we have been using the SOAP encoding in this example, we are free to use other encoding styles, such as WDDX and XML-RPC. The SOAP serialization and the messaging protocol should be considered separately, even though they are bundled into the same specification.
The SOAP encoding makes heavy reference to XML Schemas as a suggested way to define the structure of the message bodies. I use IDL instead because it is more established and more widely known. In fact, XML Schemas are not even yet a complete specification.