Any "3D object" has three images:
- that to be in the superior part (sufix:U) having Z=15 (the avatar will walk under it);
- that to be in the inferior part (sufix:D) having Z=6 (the avatar will walk over it);
- that to collide with the avatar (sufix:S) having Z=0.
Take a look of this object working at this LOT.