What is last 2 letters on the aircraft registeration such as ExpressJet Airlines Inc. dba United Express and Skywest Airlines and Mesa Airlines dba United Express?
It is usually indicative of who owns the AC.
Most expressjet ERJ’s do not have letters in their registrations and instead use 5 numbers, like Continental Airlines aircraft (who was their parent company at one time). The Expressjet CRJ’s letters often indicate their original owner or operator, with AS common for former ASA-operated aircraft and CA common for former Comair operated aircraft.
Most Mesa CRJ-700’s flying for UAX have “MJ” suffixes. I do not know the importance of the initials MJ, but these aircraft are owned by and were initially delivered to Mesa.