Create a Blanket Purchase Order
You can create a blanket purchase order by checking the Leave Open toggle on the Purchase Orders window (PO> Purchase Orders). When the Leave Open toggle is checked, the purchase order will stay open even after all of the purchase order line items have been closed. Purchase order line items will still close are they are invoiced, but the purchase order will not. This means you can add new line items to the purchase order using the Change Orders process (PO> Change Orders) even after all of the line items on the invoice have been invoiced.
If you do not check the Leave Open toggle, the purchase order will close when all of the purchase order line items have been invoiced.