Credit and debit notes regulate post-invoice tax or value adjustments and require prescribed particulars and return declaration. Where a tax invoice overstated value or tax, or goods are returned or services deficient, a registered taxable person may issue a credit note with prescribed particulars and must declare it in the return for the month of issuance (but no later than the September following the end of the year or the annual return filing), with tax liability adjusted under the Act; reduction is barred if tax incidence was passed to another person. Conversely, if a tax invoice understated value or tax, the supplier must issue a debit note (including a supplementary invoice), declare it in the monthly return, and adjust tax liability accordingly.