Credit and debit notes under GST regulate invoice adjustments, tax liability revisions, and return-based reporting requirements. Credit notes may be issued where the invoice value or tax exceeds the actual taxable value or tax payable, where goods are returned, where supplies are deficient, or where a permissible discount is given. The supplier must declare the credit note in the monthly return within the statutory time limit, and tax liability is adjusted in the prescribed manner, subject to restrictions on reduction of output tax liability. Debit notes apply where the invoice value or tax is found to be less than the actual taxable value or tax payable, and must also be declared in the monthly return with corresponding tax adjustment. A debit note includes a supplementary invoice.