Hi,
For a vendor under that Purchase Org the payment term will be unique and this will get defaulted into the PO header . Also this defaulted term can be changed during PO creation . Though it allows for a change for a PO the terms are unique .
However Paymkent terms will only come into existsnace only when the invoice is posted . Yes if the PO is having differenet line items and differrent line items are posted with differenet invoices you can have the payment terms changed to the suitable one for individual invoice posting . However when all items are posted under a single invoice this is not possible . However this is not advicible as this might create clearing issues .
Regards,