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For reference see also here https://tex.stackexchange.com/questions/520573/

I have merged both topics as they are essentially the same post.

Oh wow! According to expl3 conventions \__mhchem_arrow_options_minLength:n is an internal function with one brace argument. According to \show and the MWE it is used as a length or skip variable. It really should be named something like \l__mhchem_arrow_options_minLength_dim and be set internally wi...

Why don't you just use LaTeX's \sb or expl3's \c_math_subscript_token ? \documentclass{article} \usepackage{expl3} \usepackage{xparse} \usepackage{amssymb} \ExplSyntaxOn \tl_new:N \tl_matrix_elem_tl \tl_set:Nn \tl_matrix_elem_tl { \ensuremath { A \sb {ij} } } % or: % \tl_set:Nn \tl_matrix_elem_tl { ...

The argument is a TeX number. Anything that would be accepted by \number is a valid argument: ⋅  explicite constants, ⋅  macros that expand to a number, ⋅  count registers, LaTeX counter can used via \value , e. g.: \alphalph{\value{page}} This means replace \renewcomm...

I would need, if possible, some clarification as to what the intended purposes of local public, local internal, global public, and public function are. I take it the difference between function and variable is clear? (This is mostly a semantic difference here as in TeX everything is a macro in the ...

Two undescores for internal, one for public TL's. Not only tokenlists but all kinds of variables, even functions. Public means that those variables and functions are allowed to be used by others and ideally are documented (just like all those in interface3). Internal functions and variables are not...

Such so-called “delimited” arguments would count as weird ( w ): \documentclass{article} \usepackage{xparse} \ExplSyntaxOn \cs_new_protected:Npn \mypackage_foo:ww #1|#2* { (#1)(#2) } \NewDocumentCommand \foo {u{|}u{*}} { \mypackage_foo:ww #1|#2* } \ExplSyntaxOff \begin{document} \foo...

No in TeX/LaTeX there is no namespace. This is part of the reason for the naming convention. The other reason is readability. After getting accustomed to the convention the code is far easier to read and understand than “traditional” (La)TeX code.

The first argument to \NewDocumentCommand can only be one command sequence token and not – like in your case – an instruction to build one. This means you have to build the command sequence token before passing it to \NewDocumentCommand . Also you are not following expl3 guidelines and naming conven...