I believe this is likely to use the NSA's definition of a "US Person"
According to the National Security Agency web site, Federal law and executive order define a United States person as any of the following:
a citizen of the United States
an alien lawfully admitted for permanent residence
an unincorporated association with a substantial number of members who are citizens of the U.S. or are aliens lawfully admitted for permanent residence
What exactly does that mean? Is it literally just anybody they "believe" to be physically outside the US? Or non-US citizens?