Hiboglossi
The Most Depressing Person
Lots of European carriers file their flights using alpha numeric callsigns instead of just the flight number. Helps to avoid confusion if similar sounding flights are in the area.
Only carrier in the US that does it with any frequency is SkyWest. They will often have the same marketing number in air at the same time or close to it, and you can’t have two with the same number at once. I was once “SlyWest 737 Uniform” going DEN-TUS one day and I may have bet my Captain no one would notice me calling it “SkyWest 737 Unicorn” then some ABQ Center controller went and cost me a case of beer…
When I was at CommutAir we had call signs with letters occasionally. This was usually only done when there were 2 flights with the same number, and one or both was delayed such that they were in the air at the same time.