Close Quarters Battle (CQB) is just a general term for designating fighting in close proximity to your opponent (i.e. across the room, down the alley, from roof top to roof top, building to building). Engagement ranges are as close as contact (which really hurts...).
For that type of stuff...a lot of guys find a shorter rifle easier to move around in/around buildings. Small SMGs can "tuck in" and you can stay compact.
Typically handgun caliber rifles/SMGs would be for CQB and others would be field guns...but since these all shoot 6mm plastic bbs at roughly the same velocity it doesn't really matter.
Now, if there was rules in place where the FPS was limited based on rifle type (i.e. handguns/smg <350, 5.56<400, 7.62+<500)...that'd be kind of cool.