What is the difference between selecting Unit or Panel when casting abilities?

When casting abilities, you can choose to focus on specific units or panels. If you select Unit, the ability will target that unit upon activation. Even if the targeted unit moves beforehand, the ability will still hit with that unit at the center of its area of effect.
Selecting Panel will cause the ability to activate on that panel. If units move outside of the area of effect surrounding that panel, they will not be affected by the ability when it activates.