VC the cap on the bottle says that because in 91 they only made DOT 3 and they didnt want you to use DOT 2.
DOT 5.1 is not a racing fluid and does not need to be replaced often. Most of your new cars will use DOT 5.1, it has a higher boiling point. I know you get brake oil instead of fluid but it is DAMN expensive, but it never boils or need replacing.