This is the rendering settings:
0 = 32x = Minimal 1 = 16x = Low 2 = 8x = Normal 3 = 4x = High 4 = 2x = Very High 5 = 1x = Extreme
Where the x-factor is the number of times X-plane halves the bitmaps in each dimension. Some bitmaps are special-cased to not derez..for example, the interface bitmap is not downsampled, as this would make the text illegible.
Note: when called during startup, this appears to return '3' regardless of the actual rendering setting. Checking the dataref after the sim has fully loaded, though, the value appears to be accurate.