It tells you on the Brohn website. It is basically about the decibel level. If it is loud, it means that yes it could be pulling more air, but makes you deaf. The quieter it is, does not always mean that it has enough power to exhaust the room.
As for 6" duct work, I have only seen that with commercial units that use a remote fan unit.
As for 6" duct work, I have only seen that with commercial units that use a remote fan unit.