character string literal and string literal in standard?

Question

I am confused by these four terms:

• character string literal

• character constants

• string literal.

• multibyte character sequence

And reading this quote in C Standard:

A character string literal need not be a string (see 7.1.1), because a null character may be embedded in it by a \0 escape sequence.

What is meant by the first part ?

Answer 1

A string-literal is

• either a character string literal, e.g. "abc";
• or UTF-8 string literal, e.g. u8"abc";
• or wide string literal, e.g. L"abc".

From the standard (emphasis mine):

A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes, as in "xyz". A UTF−8 string literal is the same, except prefixed by u8. A wide string literal is the same, except prefixed by the letter L, u, or U.
....
In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence that results from a string literal or literals. ⁷⁸⁾

_{78) A string literal need not be a string (see 7.1.1), because a null character may be embedded in it by a \0 escape sequence.}

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A string is a contiguous sequence of characters terminated by and including the first null character.

So a string literal may have \0 also in the middle or even at the beginning, for instance "a\0b" or "\0ab". I think this is what the footnote is saying.

A character constant is a c-char-sequence (usually a single character) in single quotes, with a possible prefix L/u/U.

An integer character constant is a sequence of one or more multibyte characters enclosed in single-quotes, as in 'x'. A wide character constant is the same, except prefixed by the letter L, u, or U.

So the terminology is not very symmetric, IMO. E.g. wide character constant is a particular case of character constant. However both character string literal and wide string literal belong to string literals.