When a President of the United States of America dies, their funeral is a large ceremonial event. According to tradition, the body of the President will undertake one final journey on Air Force One on the way to the President's final resting place.
The plane is only called 'Air Force One,' though, when the President is on board. As Donald Trump wasn't on board when President Bush Snr was taking his final flight, the plane was referred to as 'Special Mission 41.'