Porn in japan is tecnically prohibited, The censor is a kind of a loophole to produce the sexual content. You can't show things considered obscene, but on the current legislation obscenity is simply genitalia full on display.
Thats why pornografy there is called "AV" (adult videos), technically is not pornography, its just some videos for adults. They are classified by the Film Classification and Rating Organization (Eirin) just as any other movie. Because of that, theres a lot of prohibitions, like can't use certain words, like movies with highschool girls theme, Joshikosei (女子高生) (highschool girl) cannot be used, so they use another word with different kanji but same pronunciation (女子校生) wich means girl student. Another specific thing is the censorship of the anus: It doesn't need a censorship unless it's penetrated. Japanese pornography is filled with gambiarra, like the classics tentacles, they where created as a way to pass by censorship. You can also read more JAV Categories JAV Uncensored.
Some popular studio of JAV Censored: Madonna, MediaStation, SODCreate, S-Cute,...
Source from Reddit.