Aggregation of taxes for contract PO's

Hi,

in the conceptual model of SRM in my client we're studying the use of shopping carts with items without taxes. One main question that have ocurred to me is: when generating a purchase order for a existing contract (cataloged contract in CCM), will the purchase order taxes be aggregated correctly according to the respective contract's taxes? How does this "machine" works?

Thanks in advance,

Cristiano Soares