In most cases the users will be perfectly happy with an automatically generated/assigned po_number, but in some cases they may want/need to specify one when creating an order. In order to accommodate both, we should populate the po_number field with an auto-generated value obtained from mod-orders. If the user decides to supply their own value instead, we need to verify that the value provide is valid (adheres to the prescribed format) and is unique. This validation can be done via mod-orders.
When validating a po_number, the entire po_number should be supplied, including the optional prefix and suffix.