Any rules on this site for fan fiction?

I'm sure it's been discussed here before, but I was curious if there are any rules or discouragement for a story that might be considered "fan fiction"? I ask because I was developing a story that involves the setting, but none of the characters of a commercially published work. Do you just need a disclaimer or should derivative works be avoided? I don't want to cause the site any trouble.

Thanks,
Melanie