The 'Mappings' tab can be organized in two different ways. By pressing the 'Switch base' button, you select whether you prefer to see a list of Remote Usernames above, that you will map with the Associated User(s)/Group(s) Access below, or a list of Associated User(s)/Group(s) Access that you will map with the Remote Username list below. This doesn't change the way it works, only the way it is shown. You might want to think that a certain remote username has several Active Directory groups it's associated with and thus choose to see the remote users above, or you might prefer to see, for example, a list of Active Directory users and link each of them with several remote users. You can try, and even go back and forth as you add users and decide which way works best for you. Switching the base doesn't change the users and their mapping.