Dell U3821DW 37.5" 21:9 Curved IPS Monitor (Silver) - Manual (Page 66)

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Operating the monitor
b. When connecting HDMI + USB-B to computer 1 and USB-C to computer
2:
NOTE: The USB Type-C connection currently supports video and data
transfer.
Make sure USB for HDMI is assigned to USB-B .
NOTE: As the USB Type-C port supports the DisplayPort Alternate
Mode, there is no need to set USB for USB-C.
NOTE: When connecting to different video input sources not shown
above, follow the same method to make correct settings for USB to pair
the ports.