When you start writing your story in the Review Room and such, what if it fits in more than one category?
I believe that General Fiction is the place. But not always. For example, if your story is romance and sci-fi, you would prefer sci-fi. If your story is horror and sci-fi, you may choose either category. It depends on which genre fits completely to your story. At least that's how I think it is. Unless you have a suggestion for a new section.
I would think one would put it in the section it fits in more. If it was Horror/Sci-fi, pick the one that is most relative to the story.
I'm just going to echo what's already been said - basically, just post it wherever you think it fits best.
The thing to keep in mind is that the piece will be critiqued in terms of the category it is posted under. Many elements of fiction don't depend on the category,such as character development, setting, exposition, and SPAG. But some do. For example, something posted as Horror that has no element of suspense may be a perfectly fine story in another category, but would fail as a horror story. The common elements will probably be commented upon as well, but comments with regard to specific motifs of the category would not be helpful to you, and the reviewer will have spent time that might have been more useful to you if focused in the intended category. For a story that fits in more than one category, you could always place it in the category you feel fits it the best, and prefix it with a comment that it crosses categories.