What defines a sub-panel?

An enclosure consisting of buss-bars and circuit breakers located at the service entrance to a building B. An enclosure consisting of buss-bars and circuit breakers with the fundamental purpose of distributing power to receptacles and the load

An enclosure consisting of buss-bars and circuit breakers designed to accept two sources of power and operate as a best source selector

An enclosure consisting of buss-bars and draw-out circuit breakers with the fundamental purpose of distributing power to branch distribution panels