A badge affixed to the accessory of a yukkuri to signify it is owned by a human. Unlike dog tags, the badges come in several colors, the three traditional ones being bronze, silver, and gold, usually representing its worth, both to the owner and sometimes society at large. It is generally accepted that harming a properly badged yukkuri that isn't your own is a big no no, especially the upper two ranks. Sometimes a platinum badge is used for exceptional yukkuri, but these are very, VERY few and far between.
It should be noted that most of the time, badges do not really mean anything. There are many stories in which silver even gold badged yukkuris disobey their owners (usually in the form of refreshing without permission even though they promised to not to do so) which results in serious consequences for them.
This goes both ways, of course. Just like having a badge doesn't mean a yukkuri is good natured, lack of badge likewise, doesn't mean yukkuri is mean spirited or evil. Considerable amount of stories contains yukkuris with no badges, average intelligence and still become good pets.
Worth of the badges and badge testing has been called into questioning many times. Since the tests mostly used to determine a yukkuri's intelligence level, it has been suggested by fandom that yukkuris with considerable intelligence often deceive their owners and test officials by hiding their scummy traits in order to get a badge.
Because of the value of badges, more distasteful yukkuri have caught on that being a pet equals easiness and sometimes will go out of their way to steal one.
Some authors might use badges to denote yukkuri with special roles, instead of being a pet yukkuri.
For example the flower badge in pool #1961