Tax invoice requirement: timing and documentary obligations determine when registered suppliers must issue invoices and related vouchers. Section 31 requires registered persons to issue a tax invoice for taxable goods and services at specified times, permits government notifications designating alternative documents or exceptions, authorises revised invoices within one month of registration, exempts low value supplies from tax invoice requirements subject to prescriptions, mandates bills of supply for exempt or composition supplies, prescribes receipt and refund vouchers for advance payments, and sets invoicing rules for continuous supplies and reverse charge circumstances.