I don't understand for what reason a person would need to know legal rights as to whether to exit a vehicle on command of a police officer.
I guess if you asked about your legal rights related to an officer searching you or your vehicle it would make more sense. But just asking you to step out of your vehicle doesn't mean that there is a potential rights violation imminent. Maybe the officer doesn't want to stand in the road so as a safety precaution he askes you to exit and stand on the side of the road. Who knows...could be one of many reasonable reasons that aren't related to a rights violation.
Can you be more specific?