State GST communications must include a DIN; missing DINs are invalid unless regularized within 15 working days. State GST officers must electronically generate and prominently quote a Document Identification Number (DIN) on specified communications; the DIN is a 19-character code verifiable on the departmental website. Exceptions (technical failure or urgent issuance while the officer is outside office) require written reasons and an express statement that the communication lacks a DIN. Communications without an electronically generated DIN and not covered by exceptions are invalid; exigent issuances must be regularized within 15 working days by superior approval, post-facto DIN generation, printing the DIN pro-forma and filing it.