When you load a stereoscopic image with no recognized stereoscopic format specifications, this dialog box automatically opens to allow you to specify the stereoscopic format that will be used by the decoder to load the image as a stereoscopic image.

Stereoscopic PNG format
This format is already explained on this page.

Side by Side
The right and left images are stored horizontally, one image at left and one image at right, and may have some padding pixels (separation) between them.

Over / Under
The right and left images are stored vertically, one image at top and one image at bottom, and may have some padding pixels (separation) between them.