Definition: Transfer or Indirect Transit passengers
Passengers arriving and departing on a different aircraft within 24 hours, or on the same aircraft bearing different flight numbers. They are counted TWICE: once upon arrival and once on departure.
On some flights, the flight number changes at an airport to designate the change between an inbound and outbound flight but with intermediate stops. An example is a flight from Barcelona to Hamburg where the flight continues to Frankfurt before returning to Barcelona. Where passengers for an intermediate destination continue their journey on the same aircraft in such circumstances, they should not be counted as transfer or indirect transit passengers.
Draft Glossary on Air Transport Statistics, 2001