Rating systems are a bit subjective and hard-to-define, but the lengthy explanation might be here. A summary of that:
1. Excessive use of foul language warrants an R-rating. (Using it a handful of times in a chapter is only PG-13, however. We're talking about frequent use of it.)
2. Sexual themes without actually being sexually explicit. (Mentioning sex or implying it through innuendo and vague terms is R. Actually having characters get it on and describing it? NC-17.)
3. Heavily detailed blood and gore (wherein you describe someone getting impaled in great detail) is R. Some minor violence (someone bleeding because of a non-lethal cut) is PG-13.
So, yeah. These are your average factors for determining whether or not something is "adult." If you're ever a bit unsure, think of what you might find in an R-rated movie. If you have a lot of it, chances are, your fic is also an R-rated fic.
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