1- The magazine that serializes is an H mag or is an R Rated Doujinshi with an explicit cover in the first place (Which does not mean that the series has to be an H series by default. For examples look into Dowman Sayman works)
2- The content is explicit on showing both reproductive organs and outright sexual intercourse with all the fluids that come with it.
3- Said sexual intercourse is the MAIN SUBJECT of the work having chapters dedicated ONLY to sex scenes. There are pretty plot-driven Hs tho.
That last point is the one that really separates smut from hentai. That's the reason stuff like Tsugumomo is stilll considered smut and that Twitter Comic about straight girls kissing got tagged with an H as fast as a fucking lighting when the specials came out.
Tho it is kinda a stupid separation because we have works always pushing the line. This work for example, comes FROM an H. And maintains a lot of the original spirit.
I know this isn't the kind of answer you wanted xddddd- Just wanted to write something anyway
@Raknasuu
Thanks for the response. It actually clarifies and muddles things a bit because now that I think about it. So many of the dumb stuff I've read on this site either toe the line or just dive right into it but aren't as explicitly sexual as others with just the smut or even some of the absurd ecchi tags.
One of my favorites, Prison School really shines in this sea of manga.