E-invoicing for businesses with a turnover over INR 500 crores has become mandatory since October 1, 2020, requiring the generation of invoice registration numbers (IRNs) through the Invoice Registration Portal (IRP). While B2B transactions must generate IRNs, B2C transactions do not require IRN submission, though a QR code must be self-generated and included on B2C invoices. The document also discusses the usage of QR codes and the Iris Onyx software, which streamlines e-invoicing processes.