Mature content in chanbaek fics rated M can be tricky because it isn't a monolith. Sometimes it's just graphic violence from a 'Hwarang' or 'Kingdom' AU that warrants the rating, other times it's explicit sexual content. The real interpretation starts with the tags. If I see 'Explicit Sexual Content' or 'Graphic Depictions of Violence' right there, I know what I'm getting into. But authors also use M for fics with heavy, dark themes—like non-con, major character death, or psychological horror—without necessarily being graphically descriptive. The tone of the summary matters a lot too; a dark, angsty premise often signals the mature elements are more about emotional brutality than physical acts.
Where I think people get tripped up is assuming M equals porn. It doesn't. I've read M-rated chanbaek stories that were all about the slow, agonizing disintegration of a relationship, with the mature content being the intense emotional abuse and mental unraveling. There was sex, but it was unsettling and tied to the power dynamics, not romantic. Conversely, I've skipped E-rated fluff because I wasn't in the mood for that much physical detail. The rating is a content warning, not a quality label or a simple thermometer for how 'hot' a fic is. You really have to read the tags and the author's notes, because one person's M is another's T.
My personal rule is to treat the M rating as a 'proceed with caution' sign. It tells me the author felt the material needed a content warning beyond the general audience. From there, I rely on the additional tags to decide if that caution is for gore, for sex, for dark themes, or for complex adult situations like messy breakups or addiction. It's less about interpreting the single letter and more about piecing together the whole warning system the author provided.