Originally Posted by N227UA
Look at NW, they use 752/744/742/D10/A332, while UA only use 772 and 744.
NW is phasing out the 742s in favor of 744s. It is also phasing out the D10s in favor of 332s. So that leaves you with three types: 752, 744, and 332. I believe the 752 is only used for smaller markets; UA obviously needs the capacity of larger aircraft.