Fullscreen window.
In this mode, Unity will create a window which is covering the whole screen. It will always run at desktop resolution, and other requested resolutions will be scaled up. OS UI will correctly show on top of the fullscreen window (such as IME input windows, GameCenter or Mac App Store UI, Battery warnings, etc). On OS X 10.7 and newer, it is possible to switch to other Applications using trackpad swipes or Cmd-Tab.