There are four scenarios where a credit note can be applied to a purchase order in DEAR. These are:
for individual items – when a unit price has been overcharged
for part of a shipment – when part of a shipment has either been short-shipped, or damaged goods were received
for a whole shipment – when an entire shipment was received incorrectly, or it was shipped out incorrectly
for a discount applied to an order after the invoice has been authorised.
More info can be found here:
https://dearsystems.freshdesk.com/support/solutions/articles/11000071261-purchase-credit-notes-and-refunds
