Not using iSupplier portal, What is the difference between 'Document' or 'Document or Shipment' ?

Customer don't use Oracle iSupplier portal.

In this case, How does it works to set 'Document' or 'Document or Shipment' in a Oracle Purchasing Module (Standard, BPA, Release) ?

I tested below case.

1. I create a new Standard PO and Select Acceptance Required Flag as 'Document' and sysdate as 'By'
2. I can create multiple PO Acceptance data. There also can be more than one 'accepted' check data.
3. After 1 day, I cannot receive any notification although the 'By' date is past.
4. I change the PO line / shipment level (Price and Promised date)
5. I approved the PO and then the new revision is created.