Thank you, I'm starting to get a good idea of where to start.
To clarify, the API above is from a billing software we use (ioncubed) so it was diffcult to edit the API provided.
We wanted to provide our clients an API, while the API above is an Full Admin access API
1. For Example if you look at this particular method,
http://docs.whmcs.com/API:Get_Invoices
This command can be used to obtain all the invoices.
2. The API call shows you all of our client invoices (over 1000 clients). I wanted a way for a client "A" to be able to get all of their invoices without showing them invoices from Client "B", "C",etc
3. I wanted Client "A" to be able to use a similar call which in turn would show all of the invoices for the client ID related to that particular client.
I hope I'm making some sense.