Many users of Dynamics GP use both Purchase Order Processing and Payables Management invoices. There are different transaction types for different types of transaction; the former is used when creating a PO for items such as stationery or consultancy services, and the latter for items such as rent or rates.
One issue that plagues customers is getting the invoice entered in the correct one of two windows: Enter/Match Invoices for when a PO was involved and Payables Transaction Entry for the remainder.
As of Dynamics GP 2015, it is now possible to get a warning when entering a Payables Management invoice when there is an outstanding purchase order.