Document Identification Number requirement mandatory for tax communications; manual exceptions require prior approval and timely regularisation. No income-tax communication may be issued on or after 1 October 2019 without a computer-generated Document Identification Number (DIN) quoted in the body; limited exceptions allow manual issuance only with written reasons and prior written approval of the Chief Commissioner/Director General, and manual communications must state the absence of a DIN. Non-conforming communications are invalid. Manual communications under specified exceptions must be regularised within 15 working days by uploading to the system, generating and communicating the DIN; a seven-day intimation is required to the Systems authority for functional unavailability, and pending manual notices must be uploaded by 31 October 2019.