Tax invoice requirements: distinct copy formats for goods and services and electronic reporting of invoice serial numbers via prescribed returns. Tax invoice rules require triplicate copies for goods (original for recipient, duplicate for transporter, triplicate for supplier) and duplicate copies for services (original for recipient, duplicate for supplier). Invoice serial numbers issued in a tax period must be furnished electronically via the Common Portal in the prescribed return (GSTR-1 or GSTR-1A). Where goods move in lieu of an invoice, a triplicate delivery challan with designated copies must be used and declared in the waybill; invoices not issued at removal must be issued after delivery and serials reported in Form GST ITC-04.