The PaintKit documentation can't be easier to find: its installer adds an Fsdreamteam icon in the Windows start button, with just one entry: the PaintKit documentation.
Once you'll read it, you'll understand why you might not see Frontier at KDEN: the operator IS set correctly, with 4 letters ICAO, so it has 4 points, just like United, which also has 4 points so, the selection would then depend on the airline parking code in the airport AFCAD, the ones with a matching airline code will get an additional 0.5 points, and the one with the highest score will be assigned.
Which means, on a parking with an UAL airline code but no FFT, United will *always* win against Frontier 4.5 to 4. On parking with only FFT code, Frontier will always win, and on a parking when both codes are listed, the selection will be again random.
On default airports, which don't use airline codes at all, a selection will always be random, since they both have 4 points.
In any case, all of this was already explained in the PaintKit documentation.
