Tax Law, § 639(a)(1)
A nonresident estate or trust must determine those items of income, gain, loss and deduction, entering into the definition of Federal distributable net income, which are derived from or connected with New York State sources under the applicable rules set forth in Part 132 of this Article.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 139.2