AFCAD are files that describe how runways, taxiways, gates, parkings, ground lights and even the navaids of an airport are laid out (did I forget something?

:) ).
No buildings, AFCAD does not use them.
A file AFCAD can describe bettered (one hopes) stock airports or it can be totally original (describing a fictional airport), but it's made and/or edited always (that I know of) by using the great AFCAD program from Lee Swordy.
PS
Each and every AFCAD files has to be activated in FS9, usually putting said file in in the fs9\Addon Scenery\Scenery folder.