The AC flare fittings were developed for the aviation industry in the 1930s, and have long since been superseded by AN. Today, it is rare that you will come across one of these fittings. AC fittings use a 30° flare angle, and they are most readily recognizable by a very short or non-existent straight section between the top of the thread (on the male fitting) and the major diameter of the flare. Most flare fittings of the other types have a non-threaded section at the base of the flare.
Note that the AC were talking about here has nothing to do with air conditioning. It is possible that some people may use the term "AC Flare" to refer to the SAE 45° fittings.