so adult content isnt limited to sex but also gore and violence, right

so how explicit does it have to be to be adult

like, in mature you just give a brief description, and in adult you give full detail?
as in between the zombie's head exploding to zombie's head exploding and splattering brain matter everywhere?

for violence i dont know where to draw the line, as combat is somewhat very important in my thing

The general rule I've always applied is 'graphic detail'.

If you describe a murder scene graphically (which I am not going to give an example of, but I think we all know it), then it's adult. If you say 'there's a body next to a large blood stain on the floor' and move on, it's mature.

If it's described in a level of detail that would merit a film getting an 18 rating (e.g. describing exactly how those rotten brains look/feel/taste as they land on the unfortunate during a prolonged scene of battering a zombie until it stops twitching), it's Adult. If it could happen in a 15-rated film without the camera cutting away, it's Mature.

Implication is almost always better storytelling anyway: I've known a great GM to make players hallucinate their own torture scenes far worse than anything described outright just by well-timed use of the phrase "there was a lot of pain, and it went very dark."

If the camera would cut away in a 15 or lower-rated film, describe that instead, e.g. character goes in for a fatal blow that's going to be messy -> "she takes the swing, and a reddish scatter marks the pristine snow." Is it giblets, is it ketchup? Narration leaves that to the reader's comprehension of intent and context.

thanks guys i now know where to draw the line for narration