Yeah, it would ideally be per fraction by the author's choice. In Latex notation, inline math is usually $a/b$ especially for simple fractions, because otherwise you expand the line spacing or have really small text. Block math is sometimes $$a/b$$ and sometimes $$\frac{a}{b}$$, depending on what looks cleaner. Exponents are often $x^{a/b}$ to avoid font size / layout issues. It's even fairly common to mix it within a single equation, especially with fractional exponents or fractions of fractions, e.g.:

$$\frac{ x^{2/3} + 7 }{ 4y/z + 9 }$$

This makes it easier to read: nested fractions kinda suck no matter what, but with bars on bars on bars they're even worse.

Anyway, there should at least be a way to specify which fraction notation you want. I would even suggest // for stacked-frac, but that's a comment. Microsoft Word and powerpoint have copied quite a bit of Latex in the past several years which have made them more usable for math, but their lack of fraction rendering control is one of my pet peeves.

Edited to add: Also the slash is used for things other than fractions, like group quotients, which are not conventionally written in the stacked notation.

See eg https://www.math.leidenuniv.nl/~psh/ANTproc/04psh.pdf