Central Board of Indirect Taxes and Customs (CBIC) and has directed vide Circular No. 122/41/2019-GST dated 05.11.2019, to provide that all documents issued in relation to summons, search authorisation, arrest memo, inspection notices and letters issued in the course of any enquiry, shall be issued with a computer generated Document Identificaiton Number (DIN) which shall be issued by such officer from the website cbicddm.gov.in.
Only in exceptional circumstances, the document may be issued without DIN when there are technical problems in issuing such DIN or when the concerned officer is not available. However, such document need to be regularised within 15 days.
Any communication not covered under exception and which does not bear DIN shall be considered as invalid.
Downlaod the Circular here.