Hi Chris,

You write:

>   what type of list would be the best fit-to-purpose?
>
> My major context is the need to have a reference standard (or standards)
> for (a subset of) 'standard LaTeX', together with a rationale, that
> non-LaTeX consumers of 'LaTeX documents' can refer to in describing their
> capabilities (to people or to other software).

if I may run the last thought on:

  and that can be regarded as definitive by those who write programs for
  translating LaTeX to-and-from other formats

I think a standard list of core LaTeX user level commands (from the
kernel and core packages) would be great.

Because it will not be frozen forever in time, it should be something
that is distributed with LaTeX.

                                    -- Bill