This is mostly no longer needed. A negative value for plane is the preferred way to do show objects in the background. It can still be used however when you want to rearrange objects in the same plane when using PLANE_MASTER for visual effects.
BACKGROUND_LAYER is a special high value that can be added to the regular layer of any atom.
The purpose of this value is to make an atom appear below any regular atoms, even if they share the same plane. In an isometric map for instance, HUD objects will always appear above the map, but makeing a HUD object appear behind the map was basically impossible without this feature until plane was implemented.
When using this special layer, it should be added to the layer an atom normally uses. For instance an obj should have a layer of BACKGROUND_LAYER + OBJ_LAYER.
This can be mixed with TOPDOWN_LAYER and EFFECTS_LAYER, but it will take precedence over both. Anything with BACKGROUND_LAYER will always appear below anything without it on the same plane.
Images or overlays with FLOAT_LAYER can be left alone. They will automatically have the same layer as whatever atom they are attached to.