Interjet is a LCC based in Mexico City. The airline operates A320-200s, A321-200(SL)s and A321NEOs, as well as Sukhoi Superjets.

Note that there maybe a folder named something like "amp_a321_CVT_" generated inside the Community Mods Folder after installing the AMP A321 and started the game for the first time. In case for any updates with this aircraft, delete the mentioned folder to make sure the changes to be applied.
For making the A321 really appear in AI traffic, these parameters are needed to insert into the aircraft.cfg of A321 (default path is "AMP_A321\SimObjects\AIRPLANES\AMP_Airbus_A321", NOT the aircraft.cfg of the livery)
icao_type_designator = "A321"
icao_manufacturer = "AIRBUS"
icao_model = "A-321"
icao_engine_type = "Jet"
icao_engine_count = 2
icao_WTC = "M"
And then, these parameters are needed to change as:
atc_type = "TT:ATCCOM.ATC_NAME AIRBUS.0.text"
atc_model = "TT:ATCCOM.AC_MODEL A321.0.text"