Tax Law, § 638(a)(1)
The share of an estate or trust in its distributable net income from New York State sources is the amount, if any, by which such distributable net income from New York State sources exceeds the aggregate of the shares therein of all its beneficiaries. Such share of the estate or trust and the applicable modifications are determined under Part 139 of this Article.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 138.2