Tax invoice requirement: suppliers must issue prescribed invoices or alternative documents to evidence taxable supplies and advance receipts. Section 31 requires registered suppliers to issue a tax invoice for taxable goods before or at removal or delivery, and for taxable services before or after provision within a prescribed period. The Government may notify categories where alternate documents suffice or invoices need not be issued. Exceptions include issuance of revised invoices upon registration, bills of supply for exempt or composition supplies, receipt vouchers for advance payments and refund vouchers if no supply occurs. Reverse charge recipients must issue invoices and payment vouchers when dealing with unregistered suppliers. Continuous supply and sale-or-return transactions carry specified invoicing timings.