Many database server like Postgres and Oracle use SEQUENCE to generate incremental id in databases. For hibernate to be able to generate the correct id, instead of generating 1 each time, it needs to know about this existing sequence. Liquibase will automatically convert an auto-increment="true" column to be of the type SERIAL or IDENTITY and this will create the default named sequence for the column which is <tablename>_<columnname>_seq.
For hibernate to recognize and use this sequence, it needs to be mentioned as part of the
This useful tip can be found: http://www.altuure.com/2008/10/14/hibernate-nativeid-generator-tip/