Depends on what the unit group is that your picking the random unit from. If you use pick all units within a certain area, it can be enemy, allied, your own, neutral, etc. Anything you want it to be. On the other hand, you can use pick all units owned by player in a certain area, pick all units of type in a certain area, pick all units of type owned by player in a certain area, etc. It doesn't even need to be in an area. You can pick every single one of the units matching a condition in the whole map if you wanted.