I was reading through the forums and decided to jump in :)

I think David is referring to the Event Roles for a seminar. This really confused me at first, also.

My own definition: Event Roles has nothing to do with enrollment, but is a way to manage who is serving at an event in some administrative capacity. For my organization, the event role for seminars is only relevant for trainers. While you may have multiple instructors enrolled in the course who can access the gradebook, etc, you may need to distinguish which specific trainer(s) is leading each seminar event.

So for us, we have many events going on at the same time. We need to distinguish that Instructor 1 is  teaching Event 1, and Instructor 2 is teaching Event 2, and so on.

I can't think of a scenario where we would assign an event role to a learner. You can turn off which types of event roles are included in the seminar global settings:

Site administration==>Seminars==>Global settings. The only roles we have ticked are Trainer and Editing Trainer.