On Overlay IN/OUT transitions, the Overlay element itself and all of the elements that belong to it can be animated together.
When you select an Overlay or any of it descendants, you will see all of them listed in the Transition Tree, up to the Overlay itself, but not other Overlays.
On Overlay transitions you will find the OUT to IN and IN to OUT phases. As you can expect, they match the IN and OUT actions you can trigger from the Elements Tree or the Graphic's Remote Control.
They are pretty self-explained, but anyways: OUT to IN is the animation applied to the elements when setting an Overlay IN —the entering transition. And IN to OUT is the leaving one, triggered by pressing the OUT button.
Keep in mind that switching between these phases will not change the IN/OUT status of the Overlay, it will only toggle the Animations you see on the Timeline. In the same way, previewing a phase won't change the real status either —it's just a preview.
Overlay IN/OUT transitions also have a built-in mechanism to avoid animation jumps when controlling live Graphics.
If you set an Overlay IN and then quickly set it OUT before the OUT to IN phase ends —i.e.: before all of the elements finish their entering animations—, the OUT to IN animations will be played in reverse, instead of jumping to the end of the OUT to IN and starting the IN to OUT phase.
This behavior will prevent visual glitches in the case the operator of the Graphics presses the wrong button.