PO Acceptances (document only) -- no date entered -- why are Acceptance Reminders Sent?

All our Standard and Blanket POs are setup with the field "acceptance required" as "Document" and no acceptance due date.What does leaving the date field blank do? We were under the assumption if no acceptance date, then it could not be late and we should not receive a reminder stating the acceptance is late.BUT we are receiving acceptance reminder emails for some POs and many releases, but not for all.I would like to prevent these acceptance reminders from being created.I would also like to understand why they are created for some POs and not others.  I am looking at wf_notifications