To be used when the content of the story is suitable only for individuals who are 18 or older. This may be due to the story having s*xual or violent themes repeatedly. Note that it can ONLY be used for a JP or KR* novel. For others the Adult genre will suffice.
* In South Korea, a child is counted as 1 year old the moment they are born. This tag is analogous to the Korean R-19 tag.