Often the ‘hanky code’ is mentioned. What is it? It’s a tradition in the scene that if you’re into certain types of activities you can indicate them by wearing a handkerchief in either your left or right back pockets. The color indicates the activity; the side, the preferred position (left is top, right is bottom). Some of the colors are gray for bondage (left is you like to tie, right is you like to be tied up), red for anal fisting (left fister, right fistee), and black for heavy SM (piercing, serious whipping, etc.); there are many many many more. There is a more detailed list within these info pages on here to look up more of those.
Sometimes the same thing is done with keys or handcuffs — keys on the left means you’re a top, on the right means you’re a bottom. It’s all just a way to signal your preferences in a public place.
Despite persistent rumors, there seems to be no ‘earring code’ involving earrings on the left/right/both/whatever.
And then there is something sometimes referred to as ‘Old Guard’ – this is a completely different discussion, is it, what is it, is it actually and so on. Mainly it is referred to when talking about a strict set of rules nd protocols for a submissive/slave to live by.