ctext-standard-encodings is a variable defined in `mule.el'.
Its value is

(ascii latin-jisx0201 katakana-jisx0201 latin-iso8859-1 latin-iso8859-2 latin-iso8859-3 latin-iso8859-4 greek-iso8859-7 arabic-iso8859-6 hebrew-iso8859-8 cyrillic-iso8859-5 latin-iso8859-9 chinese-gb2312 japanese-jisx0208 korean-ksc5601)

List of approved standard encodings (i.e. charsets) of X's Compound Text.
Coding-system `compound-text-with-extensions' encodes a character
belonging to any of those charsets using the normal ISO2022
designation sequence unless the current language environment or
the variable `ctext-non-standard-encodings' decide to use an extended
segment of CTEXT for that character. See also the documentation
of `ctext-non-standard-encodings-alist'.