[lex.pptoken] p2 says

Whitespace can appear within a preprocessing token only as part of a header name or between the quotation characters in a character literal or string literal.

Since we have named " as U+0022 QUOTATION MARK and named ' as U+0027 APOSTROPHE, we shouldn't use "quotation characters" to generally name such two characters. We should splict it to the term to make the meaning be clearer.

Whitespace can appear within a preprocessing token only as part of a header name or between the quotation characters U+0027 APOSTROPHE characters in a character literal or between the U+0022 QUOTATION MARK characters in a string literal.