Alphanumeric flight identification in flight plan Why do some airlines use alphanumeric callsigns in the flightplan when the flight number printed on the ticket consists of only numbers?

For example, the flight TK871 uses callsign THY4JT.
when flight delays cause a duplication of a flight number- there is a suffix added.  For example- flight 1423 DFW/OKC is also operating as OKC to DFW. Normally a turn of equipment- however, DFW/OKC delayed significantly- equipment and crew is available to operate 1423 OKC/DFW.  These 2 could be in route simultaneously- one is usually designated as a suffix- 1423J  Call sign American 1 4 2 3 Juliette