Explanation
UDIN (Unique Document Identification Number) was made mandatory by ICAI from 1 February 2019. A CA must generate a UDIN on the ICAI portal (udin.icai.org) for every document they certify, attest, or sign — including ITR acknowledgements in the CA's capacity, audit reports, certificates for banks, GST certificates, balance-sheet certifications. UDIN is valid for the document for which it is generated. If a CA has forgotten to generate a UDIN, it can be done retrospectively within 60 days of document date. The UDIN must be mentioned on the document itself. Verify on the ICAI UDIN portal.