Being a part of an organization, association, or any group is an honor that one takes at its highest, especially if that group is an elite one that not just anyone can easily join. These groups usually embarks a certain vision shared by the members. They also have different means or ways to distinguis their members from others.
One famous way is owning a challenge coin. Challenge coins date back to as old as the second world war. It was used before as a way to verify the identity of a person in a group, as spies lurks everywhere then. The challenge coins would bear special information like date that only bona fide members will have.
Challenge coins were named as such because aside from it being a mark of a true member of a group, it features a challenge to members in terms of carrying it at all times. There were also rules then like you cannot let the coin be marked or defaced in any way that is not part of the original look. The mere idea of also carrying the coin and not losing it also makes a good challenge to anyone.
Aside from being used for challenging or marking a member, these coins are also used to serve like medals. It is given to anyone who has done an extraorndinary service to the group. In a way, it is like an award given and treated with honor.
Nowadays, challenge coins are still being used by military and other groups or organizations.