VSF uses two member roles: Commander and Standby.
This is the Commander for the VSF stack. Control and management plane protocols run on the Commander, it is responsible for managing the forwarding databases, synchronizing them with the standby and controlling all line cards (including that of the standby).
Standby is a stateful backup device for the Commander and ready to take control of the VSF stack if the Commander device crashes. This enables the VSF stack to continue its operations seamlessly in the event of a failure.