> 'number    exact integer or #f
> 'symbol    symbol or #f
> 'sqlstate  5-character string or #f

This concurs with PHP's PDO
(https://www.php.net/manual/en/pdostatement.errorinfo.php) database driver:

Element	Information
0	SQLSTATE error code (a five characters alphanumeric identifier defined
in the ANSI SQL standard).
1	Driver specific error code.
2	Driver specific error message.

Except that we also have 'symbol for the proprietary C identifier.