The ability to specify VAT rates within purchase requisitions

When creating a purchase requisition the VAT rate cannot be specified; upon generating a PO from an approved requisition, the PO sets the VAT rate and VAT amount based on the supplier's default VAT rate. This causes errors on POs where suppliers provide products or services at more than one VAT rate, since the PO reverts to that supplier's default VAT rate rather than letting us apply the correct VAT rate for each item on the PO.