Removed the BelowUnits trait

as discussed in #3670. It is replaced by per sequence ZOffsets and won't conflict with it anymore.
You're muddling up the coordinate definitions here.  The z-offset units are integers specifying a render order, which happen to have the relationship that a sprite with a z-offset of 1 will have the same sorting as a sprite positioned with a world y-offset of 1.  Cells don't come into this at all, except for the relationship that 1024 world units is one cell.

By specifying this as a non-standard cell unit, converting that to a world range, and then using the above equality to convert to a z-offset you are actually introducing a magic number.