Sky Light And Directional Sky Light
Sky light is a general ambience around the map that can see the sky. Directional Sky light changes the color of the probe in a cardinal direction, an example of using it, is if I want one side of my map that is heavy in shadow to have more blue, say North, then I would set North Override to a blue color. Then if I wanted the opposite side to be Pink I would set South Override to pink.
Sky Light Properties
Directional Sky Light