Unique Identification Number rules govern Form No. 121 declarations, digitization, and quarterly filing obligations for payers. Procedure, formats and standards are prescribed for generation and allotment of a Unique Identification Number (UIN) for Part A of Form No. 121 and for quarterly furnishing of Part B by the payer. The UIN is a 26-character code made up of a sequence number, the relevant tax year, and the payer's TAN, with paper declarations to be digitized and numbered in continuation of the electronic sequence. Part B must be filed on the Income-tax e-filing portal within prescribed timelines, and quarterly statements must include the declaration particulars and UIN even where no tax has been deducted.