The complexity is because there are many transactions with a vendor where some of the goods received are returned over a period of 1 year. Thus there are refund transactions involved that need to be captured.
The PO only indicate the final overall purchase that did not take these into account. This is why users did not post installment invoices from PO itself.
When the PO has been completed, users do NOT want to delete invoiced PO as they want to keep it for record purpose and to use it to create new PO for next year repeated purchase. But they do not wish to have all these invoiced POs remain with all the new POs on the same page as it will become messy.
Will be good if NAV allowed invoiced PO to get posted into the system for archiving.