Dell U3821DW 37.5" 21:9 Curved IPS Monitor (Silver) - Manual (Page 49)

Operating the monitor
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Reset Display Resets all display settings to the factory preset
values.
PIP/PBP This function brings up a window displaying image
from another input source. Press the joystick to
confirm the selection.
NOTE: The images under PBP mode will be
displayed at the center of the split windows.
Sub-Window
Main Window
USB Type-C
DP
HDMI 1
HDMI 1
DP
USB Type-C
HDMI 2
HDMI 2