Replace Masked mode enables you to replace parts of contents coming from the pinball machine with your own scene content. Only the masked parts of the frame coming from the pinball machine are replaced with the content from the scene. The scene timing is independent from the timing of the frames coming from the pinball machine.
This mode can be useful to retheme machines with a split screen with score and animation.
To define which parts of the display should be replaced you need to enable the L-Mask checkbox and draw the areas which you want to have replaced by your own content.
Repeat that step for every frame of your scene.