Credit Note/Debit Note shall refer to the original supply that state the number and date of the original tax invoice. For example, due to damage of an item being supplied, if an original supply tax invoice’s item is “SR”, then the item in a Credit Note’s/Debit Note’s also apply “SR” as the item tax code, and GST need to be adjusted accordingly. Therefore, the Credit Note/Debit Note may use the same tax code as tax invoice.
In your case you can use TX.
Tax Code: AJP
Any adjustment made to Input Tax such as bad debt relief, and other input tax adjustments such as longer period adjustment and partial exemption adjustments. The GST registrant who claiming the Bad Debt Relief (AR only) has to declare the value of this claimed Bad Debt under field (6b), and field (17) of GST-03 return. Please refer to APPENDIX 5 for more details on specification for partial exemption adjustments. (This tax code is NOT applicable for credit/debit note).
(Note: For more information on input tax credit, disallowance input tax, out of scope purchase, please refer to Guide on Input Tax Credit. Please refer to GST legislations (GST Acts 2014, GST Regulations 2014, GST (Exempt Supply) Order 2014) and Specific Guide (Guide on Partial Exemption & Guide on Input Tax Credit) for details on apportionment and de minimis rule.)
(You may also refer to GST GUIDE ON ACCOUNTING SOFTWARE ENHANCEMENT TOWARDS GST COMPLIANCE from Customs website).