Tax invoice requirement: issued at or before supply with specified exceptions for exempt, low-value and advance-payment situations. A registered person must issue a tax invoice for taxable goods at removal or delivery and for taxable services before or after provision but within a prescribed period; the Government may notify alternative documents or exemptions. Exceptions and mechanisms include issuance of revised invoices after registration, omission of invoice for low-value supplies subject to conditions, bills of supply for exempt or composition supplies, receipt vouchers for advance payments and refund vouchers if no supply occurs. Reverse charge payers must issue invoices for inward supplies from unregistered suppliers and payment vouchers on payment; timing rules govern continuous supplies and approval sale-or-return removals.