OK, think of day mode and night mode. The taillights have a day mode (off) and a night mode (on)
The dash is forced in day mode (bright) if the lights are in day mode. If the lights are in night mode, the dash is in night mode and the dimmer controls the brightness, but you can force the dash to day mode by setting it to detent.
Now the display has a day mode and a night mode. The display can only go to night mode if the dash is in night mode (tail light on, dimmer not in detent). You can force the display to go into day mode even if the dash is in night mode by pressing Display, then pressing the soft button Day Mode. The soft button only appears if the dash is in night mode.
One caveat: Although the display's contrast and 'brightness' can be forced to day mode, the backlight still follows the dash day/night mode.