It's a bar game, that's taken seriously.
Among Warriors, you carry a challenge coin, which is a 1 3/4" dia. or so brass and enamel disc with a unit insignia on it. If a bunch are in a bar, and someone hauls out and slaps down his coin, everyone else has to show he's a member of the Frat, by meeting the Challenge to show his coin.
You don't have one, you buy the next round.
Even civilian contractors have challenge coins, with corporate logos on them. It's sort of an "I was There" symbol, when skin ink was against regulations. Faggot Lefties don't know...
proudly have 2
Hah!
Well that’s kind of cool, have to say.
We tried to have a coin created for our work with DHA. It was easier to get a CMH made.