Which Rating is Displayed

I hate to ask this, because it must have been asked before, but it didn't come up in any searches.

Why do some people show a blitz rating, some show a rapid rating, some show an online rating, etc?

Currently, my avatar shows my rapid rating, which seems reasonable enough.  However, this is neither my highest (turn based) nor my most recent (blitz) so I am wondering the criteria.

 It's the highest of the ratings that involve playing the game... i.e. no tactics or lessons or anything like that. Only live chess or daily chess.

Oh, hey, look at that.  My rapid rating actually is above my turn-based rating.  So...yeah...easy answer.  Sorry for not being more observant.