atc_type =manufacturer (ex: Douglas)
atc_model =model (ex: DC3)
Also in the Fltsim section...
atc_id= tail number (ex: N1257C, not to exceed 7 digits/letters)
atc_airline= airline (ex: Delta)
atc_flight_number= flight number (ex: 5487, not to exceed 4 digits)
The ATC will use a combination of these for ID of the aircraft.
Most normal calls from the ATC will be using this type of combination when a non-airline flight:
atc_type + last three digits/letters of the atc_id (ex: Douglas 57C)
Unless you're in a airline flight when it should be:
atc_airline + atc_flight_number (ex: Delta 5487)
If the atc_type and atc_model are left blank, the ATC will fall back to Experimental. Also atc_type can use Experimental as the manufacturer.
When first contacting center to get your squawk number, the call out should include the type, model and tail number.
Hope this helps clear the muck.