Free is typically one-liner posts with very little attention to grammar, worldbuilding, or deep characterization. There is no standard to follow, and as such, most of the people writing on Free have little time to devote to big RPs, are just starting to RP, or are -- to be blunt here -- just not the greatest writers. RP here if you don't want to worry about an application being rejected.

Casual tends to be a mixed bag of people who would be just as comfortable in Free or Advanced. For this reason, Casual is the only section where people use "High" or "Low" to describe the level of writing they're looking for, so you'll see a lot of "High Casual" tags and things like that. In my experience, this makes the level of content you'll find a crap shoot. The posts on Casual are usually much more fleshed out than Free posts, but not as in depth as advanced posts. RP here if you want to tell a story without going into fine details.

Advanced has the most worldbuilding, characterbuilding, and so on. Typically 3-4 paragraph posts, despite the board's two paragraph minimum suggestion. The posts here have the most detail, the history of the world you're using is usually described in some way, and people will call you out if your ideas don't mesh with the setting. RP here if you want the most immersive, in-depth storytelling.

1X1 is for two people to RP with one another, which tends to fall into one of two categories; People whackin' it and people falling in love. The levels here can be anywhere from Free to Advanced, depending on what the OP says. Arena tends to range from Casual to Advanced, whereas Nation is almost always Advanced.