The IRS has issued a revision of Publication 1187, Specifications for Electronic Filing of Form 1042-S. The revision addresses the use of Amendment Numbers and Unique Form Identifiers when transmitting one and two transaction corrections.
In a One Transaction Correction (used for incorrect money amounts, incorrect codes and/or check boxes, incorrect addresses, or forms 1042-S filed in error) and in Transactions 1 and 2 of a Two Transaction Correction (used for missing recipient TIN, incorrect recipient TIN, name, or name and address), the directions instruct the filer to “report the (Amendment Number) in position 998 of the ‘Q’ record” and to “report the Unique Form Identifier from the Original Form 1042-S in positions 988-997.”
Each time that you amend the same form (as determined by the unique form identifier), you must provide the amendment number in Field Position 998 (using “1” for the first amendment and increasing sequentially for each subsequent amendment).
A Unique Identifying Number is used to identify which information return is being corrected or amended when multiple information returns are filed by a withholding agent with respect to the same recipient. The unique identifying number cannot be the recipient's U.S. or foreign TIN. The unique identifying number must be numeric. The length of a given identifying number must be exactly 10 digits. The identifying number must be unique to each original Form 1042‐S filed for the current year. The identifying number can be used on a new original form in a subsequent year.