The enabled checkbox is used to allow you to share cars or transponders with other racers. While you can define multiple racers with the same UID only one racer with a shared UID may be enabled at a time. To enable a different racer that is sharing a transponder you may either click the enabled checkbox in the table showing all the racers which will then automatically save the change or you can select the individual racer in the table and then edit that racers information and click save manually. When you enable a racer the software will automatically disable any other racer that shares that UID.