26 C.F.R. § 20.2103-1
The "entire gross estate" wherever situated of a nonresident who was not a citizen of the United States at the time of his death is made up in the same way as the "gross estate" of a citizen or resident of the United States. See §§ 20.2031-1 through 20.2044-1 . See paragraphs (a) and (c) of § 20.2031-1 for the circumstances under which real property situated outside the United States is excluded from the gross estate of a citizen or resident of the United States. However, except as provided in section 2107(b) with respect to the estates of certain expatriates, in the case of a nonresident not a citizen, only that part of the entire gross estate which on the date of the decedent's death is situated in the United States is included in his taxable estate. In fact, property situated outside the United States need not be disclosed on the return unless section 2107 is applicable, certain deductions are claimed, or information is specifically requested. See §§ 20.2106-1 , 20.2106-2 , and 20.2107-1 . For a description of property considered to be situated in the United States, see § 20.2104-1 . For a description of property considered to be situated outside the United States, see § 20.2105-1 .
26 C.F.R. §20.2103-1