The designation for the Air Force
plane carrying the Vice-President
of the United States.
The Vice President never flies with the President. If anything happens to the president, like God-Forbid a plane crash, the Vice President is safe and can take charge immediately. So the President flies on the special jet Air Force One. The Vice-President gets Air Force Two.
If an Army Plane has him, it's Army Two, Navy - Navy Two, etc. If it's a private plane, it's Executive Two. It only gets that designation if the Vice-President is aboard. So if he goes skydiving, the plane changes its designation in flight. If he does that cool trick in the movie Air Force One where the people manage to transfer to the plane en route, the plane changes its designation. Same as if he's sworn in on the plane, like Lyndon Johnson.
Why? Because it's cool, and also the military is to keep a close eye on the exact whereabouts of its leaders. You don't want to be looking at a big roster of call-signs and asking each which one has your guy on board.