Invoice Reference Number requirement mandates electronic invoicing via portal for notified taxpayers; other formats not valid. Notified classes of registered persons must prepare invoices with particulars in FORM GST INV-01 and obtain an Invoice Reference Number by uploading the information on the Common GST Electronic Portal; invoices issued otherwise by those persons will not be treated as invoices, and the standard invoice provisions will not apply to invoices prepared under this electronic invoicing mechanism.