I'm glad you decided to do IEA, its so much fun! Basically, you go to a show, and you'll watch the horses that'll be in your division warm up with other riders. Then, when its your turn to draw, you pick a random horse out of a hat. Thats your horse for your division. You show on that horse, and then you get your ribbon. Its quite simple. Thats just how it is in my zone, it might be different where you live. For more questions, I recommend talking to your trainer or team captain. For division and showing details, you should look at the IEA website or ask your trainer or team captain about it.
Hope this helped!
It's more like your trainer goes to a meeting before your class, and it's drawn electronically now (I think). And it can be any horse that is in your class whether it's from your barn or not. You have flat only, which is two flat classes but only second one counts for points but u still get both ribbons, then cross rails, one flat and one jumping, 2ft, one flat and one jumping, and 2'6, one flat and one jumping. All the jumping classes count for points in both classes. They also have a chart with all the horses and their personalities and requirements, like they need a crop, or don't use spurs etc. the riders cannot adjust the girth of another barn's horse, only the stirrups.
basically you pick a horse from a bucket/hat and that is the horse you will ride for the flat and then you pick another horse for jumping. for the flat you don't get to ge a feel for the horse at all you just hop on. but for jumping you get two warm up jumps to get the feel of the horse