Tax invoice requirement: suppliers must issue invoices at or before supply, subject to prescribed exceptions and documentary rules. Suppliers must issue a tax invoice at or before removal or delivery for goods and within a prescribed period for services, showing prescribed particulars. Exempt supplies or those under section 10 require a bill of supply; small-value supplies may be excepted from invoicing. Advance payments require a receipt voucher and, if no supply follows, a refund voucher. Recipients liable under reverse charge must issue invoices and payment vouchers for supplies from unregistered suppliers. Continuous supply and goods sent on approval are governed by specified timing rules for invoicing.