History: NL had decoration, a.k.a. no-effect gadget. Deco was drawn on top of terrain every time. Some decoration even looked like terrain, e.g., trees and lampposts in L2 shadow. Very enraging, and Nepster eventually commented these tiles with "do NOT use" in their text-format conversions.
Fast-forward a year: IchoTolot clarified the tiles. Some of the confusing decoration is now animated, some others are replaced by proper terrain. Movable background has always been drawn behind the terrain, where most decoration should belong, too. This made decoration a special kind of moving background, moving at speed 0.
Moving background isn't a good name anymore, most tiles will be set still. The name is for clarity during conversion. "do NOT use" should vanish once the trees animate clearly.
-- Simon