MIeventer, great post!
I event, and my first time out to see if I even WANTED to pursue eventing training I did an unrecognized event. If you're new to eventing, definitely do this first. I did the 12'' class, (which kinnnd of felt like a joke because Lena and I were showing considerably higher at the time- and I was against a bunch of kiddies xD) to see if my horse and I were even compatible for the sport! Luckily we were fine and even won our division, but one of my friends discovered that her horse had a fear of giant rocks on her way into the DRESSAGE ARENA wayy before we even got near the cross country field. The judge had to come out of her car and lead the horse past the rock! Can you imagine if that happened at a recognized event? Actually, it wouldn't happen. The judge would just be like "oops times up to enter the arena, DQ!" Oh, and there's also a pretty big price difference between recognized and unrecognized eventing. In my MD area, unrecognized is around $80, and USEA starts around $175 for BN.
If you're planning on competing period, find a cross country course to school on before you go out to compete! Nothing prepared me better than going out and messing around on an actual USEA approved course for a day. Even though homemade cross country jumps are cute and fun to school on, they're really not the same thing- unless your barn actually invested the money in a realistic course. You should also know how your horse reacts to water because chances are, you may run into some issues your first time asking your horse to blindly plunge into a scary pond, and this is where your crop comes in handy too! Lena hates water, and one event over the summer the course was on had 2 separate water jumps. The first one, Lena skidded to a stop in front of the bank and I had to kick her before she jumped into it. The second water jump actually had a jump 2 strides before the bank, and once Lena landed she skidded to stop before that pond! Besides that I almost came off, no matter how many times I kicked her she wouldn't stop side stepping around the water, so when I finally remembered that I had been holding a crop the entire time after smacksmacksmack we were off into the water! Moral(s) of the story, always know what your horse is going to do in front of something "scary", and always cary a crop. ;)
Oh, and unrecognized events go from elementary (12'') to baby beginner novice (2'). I'm not aware of any 2'3 divisions...