PO's with tax

Hi,

In my present process.For service (ME21N)PO'S withholdingtax is calculated and for non service PO's there is no withholdingtax calculated.

In standard sap PO's.Tax will be calculated for both service POS and non service PO's.

Can any guide me where the coding is done by which the system is calculating tax on service pos and no tax calculated on material pos.

It will be great help.if you can through some light.

Thanks

Hagi