description of enterprise service message types

Hello people,

I'd like to know where to find the description of the parameters of enterpise service operations. For example the operation "Create Invoice" (Process Component: SRM) needs a lot of input parameters. I took a look into the Integration Builder from the ESR, but there was no description for the different message types.

The problem is: For example I wonder what is the difference between seller party and vendor party or what BillToID means. Without the knowledge of the meaning of the parameters I can't do an invocation.

If anybody could tell me where to find these information, this would help me very much.

BTW: Does anybody know a Test-SRM-Server that has an Invoice-ES that I could invoke?

Thanks a lot,

Michael