26 C.F.R. § 1.875-1
Whether a nonresident alien individual who is a member of a partnership is taxable in accordance with subsection (a), (b), or (c) of section 871 may depend on the status of the partnership. A nonresident alien individual who is a member of a partnership which is not engaged in trade or business within the United States is subject to the provisions of section 871 (a) or (b), as the case may be, depending on whether or not he receives during the taxable year an aggregate of more than $15,400 gross income described in section 871(a), if he is not otherwise engaged in trade or business within the United States. A nonresident alien individual who is a member of a partnership which at any time within the taxable year is engaged in trade or business within the United States is considered as being engaged in trade or business within the United States and is therefore taxable under section 871(c). For definition of what the term "partnership" includes, see section 7701(a)(2) and the regulations in part 301 of this chapter (Regulations on Procedure and Administration). The test of whether a partnership is engaged in trade or business within the United States is the same as in the case of a nonresident alien individual. See § 1.871-8 .
26 C.F.R. §1.875-1