Credit notes and debit notes under GST govern invoice corrections, tax adjustment, and disclosure timelines for suppliers. Registered suppliers may issue credit notes when the tax invoice exceeds the correct taxable value or tax payable, goods are returned, supplies are deficient, or a prescribed discount is given, and must report them within the prescribed return period subject to the stated time limit. Output tax reduction is barred where the related input tax credit has not been reversed or where tax incidence has been passed on. Debit notes must be issued where the invoice understates the taxable value or tax payable, and a debit note includes a supplementary invoice.