The AFCADs are the files that defined airport parkings and the taxiway layout.
The AFCAD downloaded by GSX (if you reply YES) are only those related to FSDT/Flightbeam airports you currently have installed. No other airports are downloaded. The download is on request, in order not to annoy users that like to customize our sceneries AFCADs only to have all their modifications overwritten during a GSX reinstall.
