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.
Bells are not completely permanent. Even certified catgirls must undergo regular review exams in order to renew their qualifications. Failure to comply or pass the review will result in revocation of the bell and its associated privileges until the catgirl passes a review, though they are allowed unlimited tries in this regard. Catgirls who have been recently approved may be required to undergo renewal exams more often.
Catgirls can choose to remove the bell at any time, though this does mean that if they are caught outside without their master and their bell is not on their person, they could be taken into custody by the police as a stray.