While 50/50 may not seem like a good "number" for a flight, there is a reason.. . 3,4,5 or 6 thousand series flight numbers are assigned to various codeshare partners of a mainline carrier. This flight is simply flight "50" for that airline, but it is coded with a 5,000 number by their major affiliate.. . For example, "Regional Jet Airlines" flying as United Express has a flight number 50. Under Uniteds timetable, all Regional Jet Airlines flights are 5xxx numbers, so the flight becomes 5050.