What is "bartop-dentistry", please?

It’s related to the dentistry notation block. It’s not part of the official Unicode block, so it’s possible that bartop-dentistry and barbottom-dentistry were intended as options as components for the other symbols or for combining/positioning with other glyphs through an OpenType feature. For example, to help with representing the old Palmer dental notation.

Those two glyphs are in the Adobe Japan1 Mapping for CID keyed fonts.