How to change Tax Code on PO?

We have recently upgraded to R12.

We have suppliers that provide us with Expense items and Goods items.

In 11i we used to change Tax Code on shipments of each line depending on Item type (VAT Expense for Expense Items, VAT Goods for Inventory Items).

How can we do the same in R12?

We don't have any move a Tax Code field in PO shipments.

Please Advise