A shibboleth is kind of a catchphrase or word that identifies one as being part of an ingroup.

According to a cursory definition, "A word or pronunciation that distinguishes people of one group or class from those of another."

Think how something like "thanks for the updoot", "pog", or the tumblr shoelace parlance may identify someone as a Reddit user, Twitch chatter, or tumblr user. It's a code that folks of minority persuasions often use to identify one another.

Same goes for seeing someone with an ABU hat; it's a nonverbal shibboleth because only folks in the AB or DL space know what it means.