Modalias format for HID is defined here:
https://elixir.bootlin.com/linux/v6.18/source/drivers/hid/hid-core.c#L2851-L2852

so it's
hid:b<bus>g<group>v<vendor>p<product>

vendor and product are kinda intuitive, they probably just copy vendor/product IDs from whatever bus the hid device is attached to.

Speaking of bus - the possible buses are defined here:

https://elixir.bootlin.com/linux/v6.18/source/include/uapi/linux/input.h#L254-L278

I think these are Linux specific identifiers.

But what is group?

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