Dell U3821DW 37.5" 21:9 Curved IPS Monitor (Silver) - Manual (Page 52)

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Operating the monitor
USB Allows you to assign the USB upstream port for the
input signals (DP, HDMI 1, or HDMI 2), thus the
monitor's USB downstream port (e.g. keyboard and
mouse) can be used by the current input signals
when you connect a computer to the upstream port.
See also
Setting the KVM switch for details.
Press the joystick to confirm the selection.
When you use only one upstream port, the
connected upstream port is active.
NOTE: To prevent data damage or loss, before
unplugging the USB upstream port, make sure that
NO USB storage devices are in use by the computer
connected to the monitor’s USB downstream port.