Why C11 define character constant recursively?

Question

The character constants are defined in c11 as:
Syntax
character-constant:
' c-char-sequence '
L' c-char-sequence '
u' c-char-sequence '
U' c-char-sequence '
c-char-sequence:
c-char
c-char-sequence c-char
c-char:
any member of the source character set except the single-quote ', backslash \, or new-line character
escape-sequence

It is defined recursively, so inside the single-quotes, there are one or more c-chars, like 'abc'.
However as I know, a character constant contains only one c-char, like 'a', doesn't it?

Characters are UTF-8 as far as I know. It can support characters beyond what ASCII can. See here http://stackoverflow.com/questions/10229156/how-many-characters-can-utf-8-encode — Dustin Nieffenegger, Apr 19 '17 at 01:24
The Macintosh File System used four bytes constants for file types and file application creator, like `'TEXT'` for a text file, `'APPL'` for an application. File name extensions were rarely used (mostly used for programming and web publishing). — curiousguy, Jul 02 '18 at 06:37

score 0 · Accepted Answer · answered Apr 19 '17 at 03:17

as I know, a character constant contains only one c-char, like 'a', doesn't it?

no, 'abcd' is also a character constant. Its value is technically implementation-defined, but everywhere I've looked it was formed out of the values of the chars, in big-endian order (in that case, 0x61626364)

The C side of cppreference has a discussion of various character constants

Why C11 define character constant recursively?

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