What determines which program is called when a voucher is selected from P0411 Form W0411G?

We have three versions of the P0411 - Supplier Ledger Inquiry.  When I select a voucher that has a PO (F0411.RPPO <> blank), the P4314 - Voucher Match Reversal is called.  When I select a voucher that does not have a PO, P0411 Form W0411A - Enter Voucher - Payment Information is called.  Where is the setup that tells it to call the P4314 - Voucher Match Reversal when there is a PO on the voucher?

Thanks in advance,

Sally