Alphanumeric flight identification in flight plan
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