This warning appears when you add an additional line item (one that was not included in the original invoice) to a credit note being created from an issued invoice.
When saving the document in such a case, a warning will appear listing the item that was not present in the invoiced document being credited.
The warning indicates that the credit note contains a line item that will not automatically include the price at which the goods were previously sold. The price will always be loaded from the price list (regardless of the price on the original invoice, which in this case is not linked).
Saving the credit note will then trigger the creation of a stock receipt (provided that automatic generation of warehouse documents is configured in the credit note type).