The layers are translucent, this allows the artist to create shading effects by removing material to allow the background layer to show through.
The number of background layers and the size of the palettes depends on the mode:
The most famous and hyped effect of Mode 7 was to turn the background layer into a planar texture map.
The game uses the mode 7 effect to allow the background layers to rotate and scale to display the course.
These background layers were later selectively blurred in the compositing application, to simulate distant views.
This forced developers to code routines to simulate multiple background layers in software, or in some cases, make do with the single plane.
The background layer contains what all the cards in a stack have in common - say, spaces for name, address and phone number.
Since buttons and fields could also exist on the background layer, but their content would differ between cards, there were s, s etc.
The erased area will convert the pixels to transparent, unless it is the background layer.
This transforms the background layer into a 2-dimensional horizontal texture-mapped plane that trades height for depth.