Tax invoice requirements: timing, content and alternative documents govern invoicing and related voucher obligations under GST. Section 31 requires a registered supplier to issue a tax invoice showing description, quantity, value and tax charged: for goods, before or at removal or delivery; for services, within a prescribed period before or after provision. It allows Government notifications on categories and alternative documents, provides exceptions for low value supplies with issuance of a bill of supply, mandates revised invoices after registration, governs advance payment receipts and refund vouchers, and prescribes timing for continuous supplies and sale or return transactions. A tax invoice includes revised invoices.