According to the IRS, you would file a 1042 S if the partnership must withhold tax on a foreign partner's distributive share of fixed or determinable annual or periodical gains and income (FDAP income) not effectively connected with a U.S. trade or business, as well as withhold on any other FDAP income paid to a foreign person regardless of whether he is a partner or not. Also a 1042 S may be reported if there is a Withholding Related to FATCA (Foreign Account Tax Compliance Act).
If the partnership has taxable income effectively connected with a U.S. trade or business that is allocable to a foreign partner, the partnership must withhold tax under IRC Section 1446. This withholding tax must be reported and paid using Forms 8804, 8805, and 8813.
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