Originally Posted by
N118NW
Airline flight schedules might have a flight 1234 that does PHX-LAX-PHX.
But the two legs might be flown by two different airplanes, so it's possible that the LAX-PHX leg might depart before the PHX-LAX leg had arrived. This would result in two flight 1234's being in the terminal and/or center airspace at the same time. When that possibility exists, one of the flights will be given a call sign like 34X, 34Z, 34M, etc, etc. It's just to avoid confusion.