Dell U3821DW 37.5" 21:9 Curved IPS Monitor (Silver) - Manual (Page 60)

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Operating the monitor
When the monitor does not support a particular resolution mode, you will see the
following message:
This means that the monitor cannot synchronize with the signal that it is receiving
from the computer. See
Monitor specifications for the Horizontal and Vertical
frequency ranges addressable by this monitor. The recommended mode is 3840 x
1600 .
You will see the following message before the DDC/CI function is disabled:
When you adjust the Brightness level above the default level for the first time, the
following message appears:
NOTE: If you select Yes, the message will not appear the next time you
intend to change the Brightness setting.