Each image in the Creatures series lies on an image plane. A plane is a number that represents the Z-order - how "far away" a thing is - in the 2D worlds of the Creatures series. Each sprite is placed on a plane, and the ordering of the planes determines how the sprites overlap one another, and how far 'forward' or 'backward' objects appear to be in the world. This is necessary because otherwise the game engine, being based on 2D sprites, would not know how to treat overlapping images. The planes are drawn in increasing number.
Another good explanation of this can be found in the Gumby Doll tutorial by Spotling.
Creatures occupy image planes 1000-3000 - take note of this when designing vehicles. If the plane is a large number (about 3000 to 4000) then it looks like the Norns walk behind the object. If the plane is a small number (about 500 to 1500) then the Norns appear to walk in front of the object. If the plane is between 1500 and 3000, then some Norns seem to walk in front, and other Norns walk behind the object.
Avoid using numbers below 500 or above 5000, because weird things start happening, like the object disappears behind the background or appears in front of the pillars. To appear correctly, things like vehicles that carry creatures must be built with a back and a front in suitable planes.