I think you're referring to the aerodrome code from ICAO annex 14. And if so then I'd expect the A320 family to be in Code letter C as they all have a 34m wingspan.
I think its used more by the planners than us. Seems to be about suitability for taxiways and gates i.e. wide enough or not.
If you're interested then the RFF categories are 6 for the 320 and 7 for the 321.