Penana doesn't have clear rules because apparently it's not a thing deemed important... Kinda stupid, if you ask me.
But good, I've asked another user to explain before. And this is the best I got. No guarantees if it's the full list, but at least a guideline.
G = kid friendly, no violence, swearing or more adult subject matter
PG= Mild violence but no gore. Further you can write about things like alcohol and tobacco and sparely use curse worth. As long as things aren't to detailed.
PG 13 - Violence and some gore. Sex scènes and adult references are allowed, but brief and non detailed.
R - Anything explicit. Things like extremely graphic violence, detailed sex scènes. Just general adult content.
Further there's no rules on what's totally not allowed (again super dumb) but I'd be careful with certain subjects like age of consent and illegal sex acts. Same goes for extremely graphic covers.
I'm unsure if those things get you banned here, but if anything it's things like that. Like it should be.