Document Identification Number requirement: all CBIC communications must quote DIN to enable online verification and authenticity. Generation and mandatory quoting of a Document Identification Number (DIN) is required on all communications (including e mails) by any CBIC office from 24.12.2019; the cbicddm.gov.in portal has been enhanced for electronic DIN generation and online verification. Communications lacking an electronically generated DIN, and not covered by exceptions in Circular No. 37/2019, shall be treated as invalid and deemed never issued unless regularised as per the stated procedure. Chief Commissioners/Director Generals must circulate these instructions and report implementation difficulties.