Common Lisp the Language, 2nd Edition
Any character whose bits and font attributes are zero may be contained
in strings. All such characters together constitute a subtype of
the characters; this subtype is called string-char.
X3J13 voted in March 1989 (CHARACTER-PROPOSAL) to eliminate the type string-char. Two new subtypes of character are base-character, defined to be equivalent to the result of the function call
and extended-character, defined to be equivalent to the type specifier
(and character (not base-character))
An implementation may support additional subtypes of character
that may or may not be supertypes of base-character.
In addition, an implementation may define base-character
to be equivalent to character. The choice of any base characters
that are not standard characters is implementation-defined.
Only base characters can be elements of a base string.
No upper bound is specified for the number of distinct characters
of type base-character-that is implementation-dependent-but the lower
bound is 96, the number of standard Common Lisp characters.