26 U.S.C. Sec. 872:

(a) General rule

In the case of a nonresident alien individual, except where the context clearly indicates otherwise, gross income includes only -

(1) gross income which is derived from sources within the United States [** the federal zone] and which is not effectively connected with the conduct of a trade or business within the United States [** federal zone], and

(2) gross income which is effectively connected with the conduct of a trade or business within the United States [** federal zone].

Copyright Chris Hansen