Overlay scenes

I have been looking at a video on youtube and seen something called overlay scenes.
I) is this possible in BGE?
II) are they doors, widows, or PORTALS into the other scene?
III) can it be done solely in blender or do I need crystal space?
IV) any good tutorials out there?

“overlay scenes” in Blender Game engine, are just a way to add “graphics” over your main one, like “scores”, for ex.
To add, or better, to change “scene”, you must use the “Set Scene” Actuator!
It’s easy to use “Set Scene”, here’s one way:
Select your “player” and add:
Collision Sensor-endoflevelproperty -> AND -> Actuator- Set Scene - Nameofthenewscene

Yeah, I think you have the name mixed up somewhere my friend.

To make windows to new scenes you need crystal space… that’s how you seamlessly add scenes.