"Coins" are memento keepsakes that seem to be increasingly common in military circles. They are generally given out in two ways:
(1) as indicting membership in something (a unit, a qualification etc). These are typically numbered; or
(2) increasingly common nowadays -- as a token, gift, keepsake etc, often as an "attaboy" award or memento for visiting a unit.
As with so many things, they are probably an Americanism seeping into common Canadian usage.
As a final point, the old Canadian Airborne Regt coin probably desrves mention . It was a type (1) coin -- awarded to an individual only when they earned their white wings (i.e. actually qualified and served in the CAR). But the really interesting thing was the coin "challenge." Having earned it, one was expected to carry it at *ALL* times. The challenge was, another qualified coin holder could produce theirs and demand that the victim produce theirs. If the demandee could not produce it, then they lost the challenge, which typically meant buying a round.
Cue anecdotes about guys doing things such as taking their coin to a jewelers shop to fix a mount so it could be hung from their dogtag chain so as to always have it, even in the showers at the shacks etc....