Minecraft wiki entry for ride, associated text: Must be one of no_ride_change, reassign_rides, and skip_riders.
If skip_riders, summons entities only for <riders: target> that are not riding on another entity.
If no_ride_change, summons entities only for <riders: target> that are not riding on and not ridden by other entity(s).
If reassign_rides or unspecified, makes <riders: target> dismount if they're riding, then summons entities for all of them.