Purchase Order Payment Terms Blank

Hi Guru's,

I am trying to understand how standard SAP works.

As per my understanding the payment term in the purchase order is derived from the Vendor Master - Purchasing Data section.  The payment term from the PO flows into the invoice during the MIRO process.  However, if the Vendor Master - Purchasing Data does not have any payment terms and so the payment terms on the PO is blank why does the invoice pick up due immediately ?

Does blank Payment Terms on the PO equal pay immediately in MIRO (invoice) ?? Or is there some kind of configuration that drives blank on the PO to equal Due immediately on the MIRO (Invoice) ?

Please advise.

Thank you

Yasmeen