U.S. source.
(B) U.S. source. The term U.S. source means derived from sources within the United States. A payment is derived from sources within the United States if it is income treated as derived from sources within the United States under sections 861 through 865 and other relevant provisions of the Code. In the case of a payment of FDAP income for which the source cannot be determined at the time of payment, see 1.1471-2(a)(5).