Tax invoice rules: timing, exceptions and alternate documents govern issuance, advance receipts, refunds and reverse charge obligations. Section 31 requires registered suppliers to issue tax invoices for goods before or at removal or delivery and for services before or within a prescribed period after provision; allows governmental notification of categories for alternative documents or exemption from invoice issuance; permits revised invoices within a limited post-registration period; prescribes substitutes (bill of supply) for exempt or composition supplies; mandates receipt vouchers for advances and refund vouchers where no supply occurs; and imposes invoice/payment voucher obligations and timing rules for reverse charge, continuous supplies, event linked payments, cessation of services, and approval for sale transactions.