Any USB device will identify itself on connection to the host computer. It does this by three numbers, the VID, the PID and the SERNO.

VID is a Vendor ID. For the micro:bit this is 0x0d28

PID is a Product ID. For the micro:bit this is 0x0204

SERNO is the serial number - this is a unique number allocated to each micro:bit 'baked in' to the nRF51 app processor.

This information is generated by the DAPLink firmware running on the KL26 interface processor on the back of the micro:bit board.

Here is a trace of a session on a Mac book air, capturing this information (Serial number edited out for anonymity)

> system_profiler SPUSBDataType > before
> system_profiler SPUSBDataType > after
> davidw$ diff before after 20a21,38
>         MBED CMSIS-DAP:
>           Product ID: 0x0204
>           Vendor ID: 0x0d28
>           Version: 10.00
>           Serial Number: XXX
>           Speed: Up to 12 Mb/sec
>           Manufacturer: MBED
>           Location ID: 0x14100000 / 5
>           Current Available (mA): 500
>           Current Required (mA): 500
>           Capacity: 8.5 MB (8,454,144 bytes)
>           Removable Media: Yes
>           Detachable Drive: Yes
>           BSD Name: disk2
>           Partition Map Type: Unknown
>           S.M.A.R.T. status: Not Supported

Keywords for search: VID, PID, whitelist, identification, ID