Document Identification Number rules for income-tax communications allow limited exceptions, post-facto approval, and system upload requirements. DIN requirements for income-tax communications apply to notices, orders, summons and similar correspondence issued to persons other than officers or authorities, and DIN may be referenced by attachment, email mention or other mode without needing every page to carry DIN. Public communications are excluded. Exceptional cases permit issue without DIN, but the communication must state the reason, obtain post-facto approval within 15 days, and be uploaded on the system within 15 working days with appropriate DIN referencing.