Roughly speaking, XeTeX (and LuaTeX) extends number of math fonts from 16 to 256, but currently (texlive 2019) latex2e only allowa math alphabets in full range 0-255, and restrict symbol fonts 0-15 (changed in this commit, in 2015).
As a XeTeX user, how can I use the full 0-255 symbol fonts?
An extension to more than 16 symbol fonts needs more extensive changes replacing all uses of
David Carlisle Mar 12 '15, from this comment
Maybe I should learn the usage of
\U... primitives, documented in
texdoc xetex, sec. 2.3.