In SC2, you can press “Alt + #” to remove the control group the unit is from and put it on a new control group. This is very useful for say, box selecting a group of harass units and splitting them into 2 groups quickly, or splitting off a single spotter unit.
Currently you have to 1. to select the group, 2. hold shift and left click each unit you want to split off, and then 3. press ctrl + # again to reset the group without the de-selected units, and then 4. press Ctrl + # to remap the new units.
All of that can be done instead by 1. box select (way faster than shift clicking) and 2. Alt + #.