content rating
A classification that rates the suitability of content for any type of media. A content rating will forewarn consumers as to any violent, sexual, or objectionable material presented in the show/game/book/etc., as well as suggesting a recommended age level suitable for experiencing this content in a mature manner. Different types of media use different rating standards, although questionable material is widely encouraged to be avoided by young children.
On Danbooru, this commonly refers to explicit doujinshi that have the R-18+ notice on the cover page, meaning the content is suitable for people 18 or older. The words "For Adult Only" can also count as a content rating. Another phrase seen on covers is 成人指定 (seijin shitei), which means "Reserved for Adults".