Subscripts vertical alignment

I would expect subscripts to straddle the baseline (though not necessarily vertically centered). That is the convention I have always seen for chemical typesetting. And how I approach them in my fonts.

The example from Plex would be more appropriate for denominators in my view, not subscripts. But I have seen other similar treatment before. So perhaps there is some other use intended in those cases.

The only really standard use of subscript numerals is in molecular formulas in chemistry, where they always straddle the baseline (partly above, partly below). This alignment is also seen for the use of subscript letters and other signs found in some kinds of scholarly publications. To my knowledge, there is no standardised use of subscript characters in which they conventionally sit on the baseline. [Denominators in diagonal fractions sit on the baseline, but these are distinct forms from subscript numerals.]

When the first set of OpenType Layout features were defined, Adobe submitted two distinct features: sinf (Scientific Inferiors) for the chemistry subscripts, and subs (Subscript) for subscripts that they imagined would be distinguished by sitting on the baseline. If fairly soon became evident that there wasn’t a practical use for the latter, and hence the sinf feature was unnecessary because the subs feature could be used for all conventional subscripts in chemistry and elsewhere. A lot of fonts do not bother to implement the sinf feature, and instead just use subs.

One way to think about this is that the superscript and subscript overlap the vertical area of the x-height in a typical text face, although not necessarily by the same amount, as they also relate in some way to cap ascender height, which is usually proportionally greater than descender depth.