Commented by: @mrotteveel

This enum is to implement DatabaseMetaData.getSQLKeywords, which specifies "Retrieves a comma-separated list of all of this database's SQL keywords that are NOT also SQL:2003 keywords." (see https://docs.oracle.com/javase/8/docs/api/java/sql/DatabaseMetaData.html#getSQLKeywords--). This is also documented on https://github.com/FirebirdSQL/jaybird/blob/master/src/main/org/firebirdsql/jdbc/FirebirdVersionMetaData.java#L93

UNKNOWN is an SQL:2003 keyword, and is therefor not included in the list. The purpose of the method is to supply reserved words in addition to the SQL standard reserved words.

Commented by: @mrotteveel

But thanks for reporting, I just noticed that the list contains a number of keywords that are also listed in SQL:2003. So I created JDBC555 to re-check the list in its entirety.