Using PCML beyond simple parameters

I have been experimenting with PCML and the ProgramCallDocument as a
nice approach to calling native RPG/CL programs on the iSeries.  Thus
far I have been passing and receiving simple parameters to my own
iSeries programs.  Now I would like to be able to retrieve multiple
items in a structure.  Although I can find lots of examples and
documentation on how to do this with system api's, I can't find anything
on what a custom RPG program should look like. For example, say my RPG
program receives a client number and returns a list of invoices and
amounts.  Can someone guide me as to what the "D" specs and the PList in
the RPG should look like and the corresponding PCML?

Thanks in advance,
Kendall