Dell UltraSharp U2424H User Manual page 72

Primary When either Auto Brightness or Auto Color Temp.
Monitor for is turned on and multiple Dell monitors that support
Sync this function are connected through MST, the
monitors will adjust their brightness or RGB settings
according to the ambient light condition detected by
the primary monitor.
NOTE: The selected monitor in Dell Display Manager
(DDM) is the primary monitor. To change the primary
monitor, select the preferred monitor in DDM. Refer
to the DDM user guide for details.
NOTE: When Auto Brightness and Auto Color
Temp. are both disabled, this function is not
available.
NOTE: If the primary or secondary monitor breaks
off from the MST, it also breaks off from the monitor
sync.
Reset Press the joystick to reset all settings under the
Brightness/ Brightness/Contrast menu to the factory preset
Contrast values.
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72 Operating the monitor