Okay, I found the manual here thanks to Mark Lord:
http://empegbbs.com/ubbthreads.php/topics/333145/Factory_tuner_manual

And I learned that the dial is a tuner-id selector in case you use your player in multiple cars and have it retain multiple settings but I don't know if this feature was ever implemented in any of the firmware releases or not so that's my question now. Does anyone know?