For movies it's like the ESRB for video games: a voluntary system, but pretty universally adhered to. I'm not sure about porn, but I skimmed this ruling and it

For movies it's like the ESRB for video games: a voluntary system, but pretty universally adhered to. I'm not sure about porn, but I skimmed this ruling and it makes frequent reference to how obscenity laws are a special case where speech may be prohibited, so it might be different.