The circular mandates that all communications such as notices, orders, summons, and letters issued to taxpayers must carry a DIN or be appropriately referenced. DIN may be included directly, via attachment, or through electronic correspondence, and need not appear on every page. Exceptions are permitted in specific situations like technical issues or lack of PAN, subject to recording reasons and obtaining post-facto approval within 15 days. Such communications must also be uploaded with DIN subsequently.