Use this tab to list the documents which amounts must be adjusted by the amount of the debit note with the Mutual Settlement Adjustment transaction type.
You can leave the Documents tab empty: in this case, the debit note itself can be used as the basis for money refund or credit (advance) for future supplies.
The "Debit Note (create)" form. The Documents tab
...
In the Business Activity column, select the business activities to which the documents are related.
In the Income Item column, select the item from the Income Items list to link the income from this adjustment.
...