DIN requirement for CBIC communications: electronic DIN must be generated, quoted, and verifiable; missing DIN renders communication invalid. The Board mandates electronic generation and quoting of a Document Identification Number (DIN) on search authorisations, summons, arrest memos, inspection notices and enquiry letters; communications without an electronically generated DIN, absent narrow exceptions for technical or urgent exigencies (with reasons recorded and an express statement), are invalid. Exigent communications must be regularised within 15 working days by post-facto superior approval, subsequent electronic DIN generation and filing. Authorised officers must be mapped and use the DDM portal with OTP verification to generate a non-editable DIN for each submission; recipients can verify DIN validity on the CBIC website. DIN format is CBIC-YYYY MM ZCDR NNNNNN.