An e-invoice is an invoice that's been registered with the government's Invoice Registration Portal, which returns an Invoice Reference Number (IRN) and a signed QR code that must appear on the printed invoice. It's mandatory once your business crosses the applicable turnover threshold.
E-invoicing applies to documents you issue yourself — Sales, and the return documents you raise (Sales Return, Purchase Return, since those are your own credit/debit notes). A regular Purchase is your supplier's document to e-invoice, not yours, so it doesn't have IRN fields.
Once generated, the IRN, acknowledgement number, acknowledgement date, and signed QR code are stored on the document and appear automatically on the printed invoice.
