The lines of content are clearly delineated in this post
and the post directly after, but it depends on what your definition of "hardcore" is - i.e., what kind of sexual activities will be taking place in the thread.
Solos if it will only take up one thread, Small Group if it takes up more.
If it includes bondage of any kind, it goes in Bondage.
If it includes non-consensual sex, it goes in NC Human (I'm guessing your characters are human).
If there's stuff that's going to squick out people, like vore, gore, bathroom play, etc., it goes in Extreme.
Also remember that if your roleplay no longer fits in the category you chose for it for one reason or another, you can always PM staff to move it.