26 C.F.R. § 2.1-3
Section 511 of the Act applies with respect to vessels operated in the foreign or domestic commerce of the United States or in the fisheries of the United States and vessels acquired or being constructed for the purpose of such operation. The foreign commerce of the United States includes commerce or trade between the United States (including the District of Columbia), the territories and possessions which are embraced within the coastwise laws, and a foreign country or other territories and possessions of the United States. The domestic commerce of the United States includes commerce or trade between ports of the United States and its territories and possessions, embraced within the coastwise laws and on inland rivers. The fisheries include the fisheries of the United States and its territories and possessions. Section 511 of the Act does not apply to vessels operated in the foreign commerce or fisheries of any country other than the United States.
26 C.F.R. § 2.1-3