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@jkbecker It's part of the tablet packaging: it's a wireless remote. It has its own USB dongle, but can connect via bluetooth too.

It's more like a numeric keypad. evtest catches it, so it's possible to customise it with a /etc/udev/hwdb.d/ rule as I described in https://www.davidrevoy.com/article989/how-to-customise-a-usb-numeric-keypad-under-gnulinux .
But it is a gadget; a good access to a full keyboard or a real numeric keypad customised might bring more possibilities of keys. I was describing the default layout an hour ago: