I am also curious but... nobody answers. :(

I suspect nobody knows. . . or everyone knows. Probably nothing in between.

I imagine it depends on the application program. In InDesign, there is a similar panel you can fill out; it overrides any choices the type designer makes. Dunno what Word does.

But then, if you are making good fractions, you should have a set of true-cut numerators and denominators, and their position IS determined by placement in the font -- just not through that panel -- at least, not in InDesign.