A character literal is exactly one graphic character between two apostrophes (LRM 15.6). The literals of an enumeration type are identifiers or character literals (LRM 5.2.2.1), so anything other than exactly one character between the apostrophes, is not a valid enumeration literal. Sigasi reports such a literal in an enumeration type declaration as an error (rule 19).
VHDL
type drive is ('L', 'H', 'HZ'); -- two characters between the apostrophes
type drive is ('L', 'H', HZ); -- an identifier, or a single character such as 'Z'