Partnerships are treated as pass-throughs or flow-throughs for 1042-S purposes (unlike for 1099 purposes). If you are making a non-U.S. source income payment (where source has been documented as non-U.S.) to a foreign partnership, the partnership can give you a Form W-8IMY to document its non-U.S. status. For documented non-U.S. source income payments, you do not need the underlying documentation on the partners to avoid withholding and reporting.
If you are making a reportable U.S. source income payment to a non-U.S. partnership or other pass-through, to avoid 30 percent withholding, you need: a Form W-8IMY with the partnership's information on it and the withholding certificates (W-8s, W-9s, etc.) on each partner, as well as a statement allocating the income among the partners.
Each partner receives a Form 1042-S (assuming they are non-U.S.) with his/her/its share of the payment reported. Withholding may be required on payments to partners where there is no claim for exemption from withholding, such as a treaty claim made on the appropriate form. If you are making a reportable U.S. source income payment to a foreign partnership or other pass-through, and you do not have documentation on the partners, the payment is treated as made to an unknown recipient. As such, it is subject to 30 percent withholding. The Form 1042-S is completed with a payee name of “unknown recipient,” not the name of the partnership; use code 21 (unknown recipient) as the recipient code on the Form 1042-S.