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IRS.gov Website
Instructions for Form 1042-S
taxmap/instr2/i1042s-016.htm#en_us_publink100047753

taxmap/instr2/i1042s-016.htm#TXMP36485dd4
Unique Form Identifier(p20)

rule
A withholding agent must provide a unique form identifier number on each Form 1042-S that it files in the box provided at the top of the form. The unique form identifier must:
If a withholding agent is filing an amended Form 1042-S, it must include the same unique form identifier that was reported by the withholding agent on the original Form 1042-S that is being amended. The unique form identifier will be 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 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.