Thanks! I had a lot of false starts too. I asked fable in my repo about your question and it said there are two schools of thought on tolerance tracking. My kernel uses exact predicates which fable says eliminates the need to track tolerances the way ACIS does. I tried to paste the full reply but my comment seemed to have been auto flagged.

I think Fable gave you a load of nonsense. Like I said, this stuff isn’t really in the training data and academic research is disconnected from commercial CG (and I’m pretty sure its wrong about ACIS - iiuc ACIS used global tolerances and its one of the reasons it lost to PS).

Robust/(adaptive) exact predicates and tolerance tracking solve two different things and a geometric kernel needs both, especially if you plan on letting models use meshes, lattice, or freeform surfaces (which is where the industry is headed for simulation driven design). The constructions you feed the exact predicates need to already account for tolerances to be correct (i.e. pcurves, vertices, etc) and the second you do any math on them - before feeding it to the predicate - you need tracking. Predicates don’t eliminate the floating point math.

(Grain of salt and all that, I don’t really know what I’m talking about)