Under Treasury Reg. Section 20.0-1(a)(b), U.S. Estate Tax is imposed on either a citizen of the U.S. or an Estate Tax Resident (i.e., a “resident” decedent is a decedent who at the time of his death, had his domicile in the U.S.)
Treasury Regulations Section 20.0-1
(b) SCOPE OF REGULATIONS
(1) ESTATES OF CITIZENS OR RESIDENTS. Subchapter A of Chapter 11 of the Code pertains to the taxation of the estate of a person who was a citizen or a resident of the United States at the time of his death. A “resident” decedent is a decedent who, at the time of his death, had his domicile in the United States. The term “United States”, as used in the estate tax regulations, includes only the States and the District of Columbia. The term also includes the Territories of Alaska and Hawaii prior to their admission as States. See section 7701(a)(9). A person acquires a domicile in a place by living there, for even a brief period of time, with no definite present intention of later removing therefrom. Residence without the requisite intention to remain indefinitely will not suffice to constitute domicile, nor will intention to change domicile effect such a change unless accompanied by actual removal. For the meaning of the term “citizen of the United States” as applied in a case where the decedent was a resident of a possession of the United States, see Section 20.2208-1. The regulations pursuant to subchapter A are set forth in Sections 20.2001-1 to 20.2056(d)-1.
(2) ESTATES OF NONRESIDENTS NOT CITIZENS. Subchapter B of Chapter 11 of the Code pertains to the taxation of the estate of a person who was a nonresident not a citizen of the United States at the time of his death. A “nonresident” decedent is a decedent who, at the time of his death, had his domicile outside the United States under the principles set forth in subparagraph (1) of this paragraph. (See, however, section 2202 with respect to missionaries in foreign service.) The regulations pursuant to Subchapter B are set forth in Sections 20.2101-1 to 20.2107-1.