When a heating system unit is rated at 80%, just exactly what does that mean as far as the AFUE rating is concerned?

How exactly does a humidifier unit work in a central heating system? Is there a water hook-up of some kind involved and where would it go?

Last, the central heating unit is being placed in the attic which is insulated, the insulating material I have no info as to what it is. Since the unit is being placed there, and it's one of those forced air kind, where is the air coming from - would it come from the attic with all that insulating material? If so, what's the possibility of some of that insulating material particle being sucked into the unit and expelled to the rooms below?

regards