26 C.F.R. § 20.2056(c)-3
The expression "passed from the decedent to a person other than his surviving spouse" refers to any property interest which, under the definition stated in § 20.2056(c)-1 is considered as having "passed from the decedent" and which under the rules referred to in § 20.2056(c)-2 is not considered as having "passed from the decedent to his surviving spouse." Interests which passed to a person other than the surviving spouse include interests so passing under the decedent's exercise, release, or nonexercise of a nontaxable power to appoint. It is immaterial whether the property interest which passed from the decedent to a person other than his surviving spouse is included in the decedent's gross estate. The term "person other than his surviving spouse" includes the possible unascertained takers of a property interest, as, for example, the members of a class to be ascertained in the future. As another example, assume that the decedent created a power of appointment over a property interest, which does not come within the purview of § 20.2056(b)-5 or § 20.2056(b)-6 . In such a case, the term "person other than his surviving spouse" refers to the possible appointees and possible takers in default (other than the spouse) of such property interest. Whether or not there is a possibility that the "person other than his surviving spouse" (or the heirs or assigns of such person) may possess or enjoy the property following termination or failure of the interest therein which passed from the decedent to his surviving spouse is to be determined as of the time of the decedent's death.
26 C.F.R. §20.2056(c)-3