If a foreign partnership is not considered to be a withholding partnership, the partners are considered to be the payees for purposes of determining what withholding rules apply. If one of the partners is also a flow-through entity, you would need to determine who the owners or beneficiaries of that entity are, and apply the withholding rules to them.
For example, you make a payment to a non-withholding foreign partnership that has three partners: a nonresident alien individual, a foreign corporation, and a U.S. individual. The partnership gives you Form W-8IMY, and associates a Form W-8BEN with the nonresident alien individual and the foreign corporation, and a Form W-9 from the U.S. individual. You would withhold the 30% tax on the amounts corresponding to the nonresident alien and the foreign corporation, and pay and report the tax on Form 1042-S. The amount of the payment corresponding to the U.S. individual would be reported on the applicable Form 1099.
Now assume that you make a payment to a non-withholding foreign partnership that has two partners: a foreign corporation and another non-withholding foreign partnership. This second partnership has two partners, both of whom are nonresident alien individuals. The first partnership gives you Form W-8IMY, associating the foreign corporation with a Form W-8BEN and associating the second partnership with its own Form W-8IMY. This second Form W-8IMY has withholding statements for its two nonresident alien partners. Since you have documentation for the withholding status of the ultimate payees, who are the foreign corporation and the two nonresident alien individuals in this case, you treat them as the payees and withhold tax from the payment to them.
Foreign Trust
A foreign trust could be a simple trust or a grantor trust. A simple trust is required to distribute all its income annually. In this case, the payees are the beneficiaries of the trust. In a grantor trust, the payees are the owners of the trust. If you are making a payment to a foreign trust, you would apply basically the same criteria as you would for a foreign partnership in determining who the beneficial owners are, what withholding tax rules apply, and how the payment should be reported. The trust should give you a Form W-8IMY or other documentation associating the withholding requirements with the payees.