The main requirement to be called Ryona is physical interaction. It doesn't have to be snuff or gore, but those categories can be side by side with Ryona. It is like the difference of Oyster and Haru Tanuki-ya. Both have physical harm, but one actually pushes the violence further.
This is if a female character is physically or mentally harmed, but the most common scenario is that it's a female character who is beaten up. So like two characters fighting and there is a female character in that fight, getting beaten, from what I understand, that's Ryona