The armbar is one of those fun techniques that can be practiced many different ways, from many different angles. In fact, the more ways one learns it, the more opportunities one sees to apply it and the more one begins to understand the underlying principle behind all instances of the technique.
Three basic positions to learn the move from are: the mount, the guard and cross body. Are these the only starting positions that one can apply the submission from? No way – but you’ve got to start somewhere. Practice all three a few times before every grappling session and pretty soon you’ll be seeing more and more angles from which to achieve the armbar.