You can use Tag Categories to create credit note reasons. You can then apply these when creating credit notes within the Timesheet Adjustment Wizard or Invoice List. There is also a column within the Credit Note List showing the reason.
Setting up credit note reasons
Go to your System Cog in the top right-hand corner, and within the General panel select Tag Categories.
You need to enter in the name field CreditNoteReasons , set Library Type to Custom and the Library values field will appear. Enter the reasons you wish to have within your dropdown selection within this field. Each reason must be separated with a comma , Click save. These reasons can be added, edited or removed as required.
Selecting your credit note reason when using the Timesheet Adjustment Wizard
to To enable the Reason for Crediting you must have Process for Invoicing ticked.
...