CoarseLevelOptions

The whole point of the level is that it arrives before the model, so it is measured in kilobytes rather than in fidelity: at 40 a five-metre car resolves to about twelve centimetres, which is a recognisable car for a few hundred kilobytes against seventy megabytes of file.

This is what lets it stay on screen while the real parts land on top of it, which is the reveal §4.6 describes: an outline, then its elements. A corner of the boundary sits at the average of the surface around it, so it is already close; this is the margin that keeps it from landing in the same plane as the part covering it and showing through in patches. Being a little inside the model costs nothing, because the model is about to cover it.