To control a unit, you must use the control unit button. There is special stuff to control the units once you select them with a mouse button or pressing g. One is a waypoint system by pressing ctrl and pressing on a place while pressing it. Another special button in game is by pressing c, it will remove all selected soldiers from the GUI. One last special feature is by dragging the mouse when selecting a place to where they go will either spread them out or put them in a group, it can be used on waypoints. To launch a missle, open gates quicker, to construct a construction yard, or any other general thing, press n to either launch/open or not launch/close or just press n for in general things.
The control units feature doesn't mean it only controls stuff like soldiers or tanks, but it can control buildings. This is useful for launching missiles, selling buildings, and by even looking into the researching gui. You can access it by clicking on a building when all the units are cleared and you can now control what happens in YOUR building.