This property can only have three states:
Some watches (Moto 360, LG G Watch R) have an “always on” display. To save battery the always on mode shows a “dimmed” watch face that usually has less info than the watch face in active mode.
You should always make sure your watch face is less demanding and using darker colors in dimmed mode. The “Never” display mode allows an overlay to be visible (on Moto 360) but able to accept touch interactions.