A nonstop flight takes off from one runway and does not land until it reaches its final destination. A direct flight refers to a flight that does not require a plane change, but does stop along the way to its destination, sometimes more than once.
systems use a hierarchy. Nonstop flights get top priority,
followed by direct flights, followed by “through” flights
(which use one flight number, but involve a change of
planes), followed by connecting flights.