The way I see it, the sections are based on two things: Time and Ability.

Free consists of two different types of RPs, from what I've seen. The first is the speed posting RP, which we are all familiar with. The other is someone's attempt to put as much information into a single post as they can, often only sizing up to about a paragraph per post. I'm in an RP of the former variety on another site, and it is quite enjoyable when I actually have the time to jump in.

Casual consists of those RPs where people who can write, or have good imaginations, go to RP. It's for those people who don't have all the free time in the world, or those who like a little bit of volume in their posts, or for those (such as myself) who are not writers by nature. I'm much more familiar and comfortable with numbers than words, and that's just who I am.

Advanced has a bunch of people who either have little time, and are unable to frequently post, yet enjoy great volume in their posts. It is also for those who have that knack for writing- for whom creating stories and putting words together to form beautiful stories comes naturally. I wish I had such an ability, and I've always been envious of those who can, but words just don't flow for me like they do for others. That's just the way it is.

The section you tend to be in speaks volumes about you as a person, when you get down to it.