If you are in the Ultimate Arena (UA) some of the people in the Realm Arena (RA) rank below 10 (and therefore not in the UA) can attack you if they can 'see' you in the RA. You can attack any one that you can 'see' in the UA. If you attack a player on another Realm and win you swap UA ranks with him and your Realm Arena rank does not change. If you attack a player from your own Realm then you swap both the UA and RA ranks. The same happens if you are attack and the other player wins, of course.

This becomes very confusing because you can have some one that is ranked 1 in the RA but 20 in the UA and at the same time some one that is ranked 10 in the RA but 7 in the UA. It all depends on how they moved within the UA.