The rules seem vague on this. I've seen a few threads on the forum labeled 18+, either because the topic is explicit or because it concerns a fanwork with explicit material, but its difficult to tell what the boundaries are.
Images with explicit content is not allowed, outside explicitly stated exceptions. Well, the rules would naturally be the place to explicitly state those exceptions... What are they?
The code of conduct also discourages NSFW text outside designated areas, but fails to explain what those areas are. I presume they just mean any thread marked 18+ by the OP, but it's difficult to tell if such threads are allowed across the entire forum, or only certain subforums. Of all the rules/guidelines, I think these ones should be as clear as possible, it's not really the place to leave it up to vibes and a case-by-case basis.
