sap web service parameter specs

Hi,

I am trying to consume some of sap webservices, namely

PurchaseOrderCreateWSD and ProductListWSD.

Both of them takes as input some parameters. And if this input does not follow the specs of the parameters, there is error.

for example, PurchaseOrderCreateWSD has a parameter, ProductID. An example is "PS-110-30A" ..Anything else like "notebook" will not work. So where do I get the specs or the list of acceptable inputs for them ?

In fact this leads to a more generic question, how/where do I find the specs of the input parameters for any SAP web service.

Thnx a lot,

Mayukh