Document Identification Number requirement: communications to taxpayers must quote DIN/RFN or be treated invalid; limited post-facto regularisation allowed. Officers must electronically generate and prominently quote a Document Identification Number (DIN)/Reference Number (RFN) on specified offline communications (search authorisations, summons, arrest memos, inspection notices and letters) issued in the course of enquiries; GST backend communications sent via the GSTN Portal are exempt, but manually issued communications must carry a DIN/RFN. Exceptions for technical difficulty or urgent offsite issuance require written reasons and an express statement; communications without DIN/RFN outside these exceptions are invalid and must be regularised within a limited working-day period by post-facto approval, electronic generation of DIN/RFN and filing the pro-forma. VAT RFN verification is available on the DVAT Portal.