The Rubber sheet tool allows users to take an image or 3D model on the phone and lay it over the imagery/map of ATAK/CivTAK.
If the map is approximately a representation of the geography, the image/model can be adjusted so the coordinates of the image that match the underlying map. If the proper settings are active, the resulting rubber sheet can be turned on and off and the transparency can e set by clicking the outline. as can KMZs placed in the GRG directory.
To turn on and off the rubber sheet display or change the transparency, you must enable outlines and (something else).
Once the rubber sheet is where you want it, you can go into the Rubber Sheet Overlay menu and click the "export button" (next to delete). This will allow you to export the rubbersheet as an Image Overlay (Previouy referred to as a GRG. The Image overlay has all the benefits of a Rubber Sheet, but can't accidentally be nudged or moved.
Below is a simple video demonstrating Rubber Sheeting of a photo and its conversion to an Image Overlay: