| Back texture: | The texture you see when you can look through all layers in front of it. |
|---|---|
| Back tiles: | The tile-based background. |
| Interactive tiles: | Fixed tiles you can't go through. Used at the edges of 'walled' areas. These are lit. |
| Interactive objects: | Gish, enemies, vehicles, moving platforms, etc. Simply where everything happens. Also lit. |
| Front tiles: | Stuff in front of you. Used for decoration (like tubes at the Sewer), to fill 'walled' areas, for some tunnels and to hide secrets. |