Credit and debit notes allow suppliers to adjust tax liability for corrected invoices, subject to prescribed return timelines. Credit and debit notes permit suppliers to correct overstated or understated tax invoices: suppliers may issue a credit note where taxable value or tax is excessive, or where goods are returned or deficient, and must declare it in the return for the month issued or by the prescribed annual cut-off, with tax liability adjusted as prescribed (subject to restriction if tax incidence has been passed to another). Suppliers must issue debit notes, including supplementary invoices, where invoices understate taxable value or tax, and declare them in the return for the month of issuance with prescribed adjustment of tax liability.