Chastity device discussions and reviews discussion

hi there, I have a question. What does the (circle) mean? Is it referring to the anal area? What exactly is that (circle), and what do we call it? Also, is the size comfortable for most people? does anyone know how to measure this specific size properly? Thanks! kgMannHinten350.jpg Ns_080109_002a.JPG KG-120-120-silber.jpg