What is a preferred charset?

[Stephen Berman]

The discussion in bug#33445 made me realize that I don't know what
distinguishes a preferred charset from other charsets and why this term
is used in `describe-char' (and no where else in Emacs that I could
find, aside from a comment in lisp/org/ox-ascii.el).  For example,
etc/HELLO uses the non-standard text/enriched annotation "x-charset" to
make `describe-char' show "latin-iso8859-1" as the preferred charset of
INVERTED EXCLAMATION MARK (#xa1), whereas when I use `C-x 8' to enter
that character in a buffer `describe-char' says its preferred charset is
"unicode".  Why are there different preferred charsets in these cases
and what's the significance and use of that difference in general, and
should it be documented?

Steve Berman