On the 15th I flew BOS-ORD on UA 881, a 757. Throughout the trip the plane was referred to as "United 881 Charlie" by ATC. What's with the "Charlie" handle there? (I thought a 757 was "heavy" but maybe it isn't big enough?)

thanks for any answers!

BTW, ch. 9 was in operation on all 4 segments of that trip, and even my wife was enjoying listening in