In the planned purchase order form, it's possible to assign a planned purchase order to an existing purchase order. When the user then clicks "Firm" the planned order is added as a new line in the existing PO instead of creating a new PO number. This only works if the delivery date is later than the original PO date and if the PO is in "Approved" status. If it is in confirmed status, the user receives an update error and the firming fails. We use this logic a lot and have to make a customization to set the PO back to approved status before we can firm that order.