Intent of MathML-Core B.3 is unclear

The mathml-core Pull Request w3c/mathml-core#302 makes some minor fixes to the tables in appendix B.3 but the initial paragraph of the appendix does not make the intent of the tables clear.

https://www.w3.org/TR/mathml-core/#combining-character-equivalences

For example grave (all accent mappings are similar):

The first table ("combining") maps the spacing grave U+0060 to the combining grave U+0300

The second table ("non combining") maps U+0300 to U+0060

It isn't clear why there are two tables, if the intended effect is to specify that either U+0300 or U+0060 may be used with <mover> to construct a grave accent.

As recent tests show, for example at

https://mathfonts.github.io/math-accents

currently which characters work as accents, especially as stretchy accents, is highly font and viewer dependent. It would be good to point to this appendix B.3 to give guidance on which characters are intended to work, but the text here is very unclear.

@fred-wang as I commented in the PR I couldn't find the script that made these tables (although clearly they relate to <combref and <noncombref entries in unicode.xml so we must have had some xslt or python once?)

This corresponds to issue w3c/mathml#247 on the full spec.