By default, in XTRF, credit notes can be used for:
correcting an invoicing mistake.
making a refund.
providing a post-sale discount.
Go to the Invoicing module → Client Invoices tab, select the checkbox for a Sent invoice(s) in question, and click the Issue a Credit Note button that appears on top of the invoices table. Alternatively, open a Sent invoice in edit mode and click the Issue a Credit Note button in the Type section. The Issue a Credit Note pop-up window appears.
From the Reason for Correction drop-down list, select the reason for issuing a credit note:
When you issue a credit note, the original invoice gets partially locked for editing, and you can’t change its status until the credit note is either sent or deleted.
You can issue only one credit note per invoice. You can’t issue a credit note for another credit note.