It depends on both the airline and the airport, and sometime the size of the aircraft/J cabin. Some airports have mini buses for F/J passengers, so zone boarding means you have to be at the gate when they call Zone 1 or J/F or priority or whatever to use that mini bus. For airports using large buses only, if the J cabin is huge, then priority boarding means getting on the first bus. If the J cabin is small, then that means getting on the bus first, which means getting off the bus last, board the plane last.