More generally, are there patterns to the naming conventions in community AMIs?
There are not any set patterns. Some vendors whom release multiple AMI's may adopt some kind of pattern to differentiate between each of them.
I suspect in the case of Redhat, the options exist due to the way its licensed. Its a non-free linux distribution that has a higher hourly rate than most other distributions. If you already have a Redhat Enterprise subscription, you can use that instead of paying the increased hourly rate.