If you are logged into Adagio with a user ID other than SYS, this would be defined in the security Groups. If you cannot edit a particular field within a new document, this is the entry template. If you are editing a credit not that has already been posted, you cannot retro-actively change the tax group.

Are you using Ship-to codes on your credit note? Tax Groups are defined there and this may be over-riding the tax group from your customer.