A bell (also known as Independent Action Permit) is an accessory worn by catgirls as a symbol of independence, meaning the bearer can go to other places without their master/owner. Earning a bell is not an easy task, as Cinnamon explains that only ten percent of catgirls who take the required social skills test pass.
The bell can be substituted in place of a catgirl's identification papers, allowing her to enter establishments and events, even where pets aren't allowed. It has a GPS feature, adding an extra layer of peace-of-mind for the catgirl's owner. There are two known colors of bells - silver and gold - which at first glance seem to indicate the wearer's level of qualification. However, as Coconut wears a silver bell despite getting full marks for her proficiency test, it is more likely to be a matter of preference.