Levels in games using the Wolfenstein 3D engine usually consist of two or more planes. Other TED5 games by id Software and Apogee Software also feature multiple planes. Each plane is a 2-dimensional, tile-based grid. Wolf3D games and mods commonly use a fixed 64x64 grid. Rise of the Triad and a few mods make use of a 128x128 grid.
TED5 is referring to the first plane as background, second plane as foreground and the third plane as info plane. This is due to the tool having been created for id Software's platformers and originated from the development on Dangerous Dave in 1988.
In Wolf3D editors and related communities the first plane is usually referred to as walls and floors, the second as objects. While Commander Keen and Rise of the Triad use 3 planes, most Wolf3D related content is using only 2.
Since 2002 the third plane (info plane) has been often used in Wolf3D mods, almost exclusively, for floor and ceiling textures. Several editors are fixed on this concept. There are very few mods go beyond 3 planes.