The Network Flash Drive is used in most Matter Network machines, as a destination filter. It holds the IDs (position) of all tagged machines. You can then put the flash drive in the destination filter in the config page of the machine in question. After that all messages or tasks send from that machine will only be accepted by the machines with IDs stored in the filter USB drive.

To tag a machine with the USB drive, just Shift Right Click it, while holding the flash drive in your hand.
To remove a machine from the list, just Shift Right Click the machine while holding the USB drive.