rev is revision, yes. It is a revision of that particular cios, maybe made with another base ios or something IDK/C. You can change your cios at anytime by installing the one you want to have over the existing one.
you have downloaded 2 versions of the same ios (19+20), when you install them both you will only have whichever one you install last.
nightstah is pretty patient but some of your questions are testing!