RE: The difference between certain plane and flying categories...
A bit more about the term "Fun Fly" - A "Fun Fly" is an event. The club advertises it, people from nearby (maybe from quite a distance) come to fly, have fun, and maybe compete in some mild competitions, like timed landings, spot landings, under the Limbo stick, and there are a lot more events that can be put on. The prize is usually a trophy or plaque or something goofy.
Anyway, then a plane that's good for "Fun Fly" competition is one that's easy to fly and land. Your speed plane or Warbird might not fit that description, and maybe not your trainer, but a Four-Star would.
There are probably other ways to look at this, and some of the other categories.
Good luck,
Dave Olson