You can have up to 9 units occupy the same space at the same time. To organize them, you may click on the individual units listed (when you click on the space the group occupies). You will know a unit is selected by the red border around the unit. In this manner, you may be very selective on exactly which units from that stack you wish to have move or attack. On the other hand, you may wish to easily select all units. This can be done by clicking the pad lock symbol to the right of the stacked units display. This will either select all units (pad lock is locked) or deselect all but one unit (pad lock is unlocked). Hope this helps.