On many services - gambling and others - every user gets unique payment address.

According to this doc - https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_Calls_list
i can do it with getnewaddress.
Also i can get total amount - getbalance.

And this command -
sendfrom    [minconf=1] [comment] [comment-to]

What is withdrawal logic ? It takes money from random addressess if only total amount allows to do its operation?