Simplify the following text for a Grade 8 native English-speaking person.
Original: They are only called Air Force One when the President is on board ; if the Vice President of another senior official is flying it is called Air Force Two .
Simplified: Air Force One is the official air traffic control call sign of any United States Air Force aircraft carrying the President of the United States . In common parlance the term refers to those Air Force aircraft whose primary mission is to transport the president ; however , any U.S. Air Force aircraft may carry the `` Air Force One '' call sign while the president is on board .