The Assistant Regional Manager, also known as Number 2, is the lower position before the Regional Manager position in Dunder Mifflin. It comes with a pay raise and more abilities. Dwight Schrute is tricked into thinking that he is Assistant Regional Manager when Michael says that he is Assistant to the Regional Manager throughtout the show. This is often referenced throughout the show's nine seasons.
In The Job, Michael leaves his Regional Manager job to take a job at Corporate. Dwight becomes the Regional Manager, and Andy becomes the official Assistant Regional Manager. However, Dwight goes behind everyone's backs and offers Pam the Secret Assistant to the Regional Manager. Michael eventually withdraws his name from the corporate job and returns to be manager of the Scranton branch. Everyone goes back to their respective positions prior to Michael leaving.
In the episode A.A.R.M, Jim becomes Assistant to the Regional Manager (Dwight), after he takes the Regional Manager position. In this episode, they look for anyone hiring to be an Assistant to the Assistant to the Regional Manager. Dwight takes the position of that as well.