Hi Alan,
Welcome to Talkgraphics.
Very good question. I had to scratch my head and try to think of a way to describe how the overlay works.
A normal frame in an animation obscures previous frames. If you want objects in a previous frame to be visible in a new frame you could copy and paste in place. Or, you can put a tick in the check box to make the new frame an overlay. The objects of the previous frame are visible as being under any objects in the new frame that overlap previous objects.
I hope that doesn't sound too obscure.
I've attached a simple animation .xar file that shows what I tried to say in words. Open the Frame Gallery if it isn't open when you load the file. Select the 3rd frame to see a yellow and a red star. The yellow star is actually only on the 2nd frame of the animation.
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