So imagine NSFW (or 18+) as a circle and sexual content as a smaller circle within it. Some people look for NSFW (say, combat footage), but not porn. If a post was designated within "sexual content", it would also put it within the realm of NSFW, so it won't show up to someone who doesn't want to see NSFW, but if someone is fine with NSFW but not sexual content, they won't see porn, but will be able to see NSFW.