Invoice Reference Number rule: registered taxpayers must obtain IRN via FORM GST INV-01 within prescribed time or invoice invalid. Registered persons who prepared tax invoices otherwise than as required under sub rule (4) of rule 48 must, for invoices issued during 1-31 October 2020, obtain an Invoice Reference Number (IRN) by uploading specified particulars in FORM GST INV-01 on the Common GST Electronic Portal within thirty days from the invoice date, failing which the document shall not be treated as an invoice; the notification is deemed issued from 1 October 2020.