Adrianne, RATI is the instinct test, and when this is titling, it will be an optional class, needing just one pass if you decide to do it. It's a set course, and they have the 3 tubes (one empty, one with rat litter, and one with a rat) out in the open for the dogs to investigate.
RATN is Novice, and that will be the first level of competition, which will involve actually having a rat hidden, on a course designed by the judge, for the dogs to find.
RATO is Open, and involves finding more than one rat. There are other differences, but those are the basics.
I entered mine in RATI, so they can easily get an idea what we are there for, and so I can hopefully figure out what their cues are for finding a rat. And in RATN, because I figured RATI didn't sound very exciting, and I wanted to see what it's like as the actual sport. I didn't try RATO, because I have no idea if they'll even be interested, I figured RATO would probably be best for dogs that have done earthdog before, and already know how to look for rats.
I hope mine want to play!