Tax invoice requirements under GST: invoicing rules determine timing, mandatory contents, and documents for input tax credit entitlement. The document sets out GST invoicing rules: a tax invoice or bill of supply is required for taxable supplies, identifies who must issue which document, and makes the invoice central to evidencing supply and claiming Input Tax Credit. It prescribes timing for issuance for goods and services, contents mandatory under the Invoice Rules, special documents (receipt vouchers, refund vouchers, revised invoices, credit/debit notes, ISD invoices), delivery challan use and triplicate/duplicate preparation, and procedural requirements for declaration in returns.