I know the BB Code very well, but I only use it for AG. Straight up simple question, is the BB Code ONLY for AG. Like can I use it on Facebook, YouTube, etc...
If those sites have BB code, yes. I don't know if they do or not, so that's something that you're going to have to find out for yourself. But just because you see something on one site doesn't mean that it'll work on another. For example, Kongregate uses textile, but that doesn't work on here since Armor Games doesn't support textile.
As GhostOfMatrix has said already if a site has a certain type of BB code enabled on their site then you can use that BB code on that site. BB code is not exclusive to Armor Games, in fact Armor Games forums actually cannot use every type of BB code that exists(such as the BB code that changes the color of your text for instance). Each site has it's own limitations so it would difficult to determine if a certain BB code would work on any particular site unless it has already been tested or announced. Also I'm kind of glad that we don't use textile as it looks like you could easily bold or italicize something on accident using that format from what I can tell.