In case anyone is looking for answers and ends up finding this thread, I finally found out what this key is for. Nothing. Apparently it was supposed to be AltGr but the Matrix guys never bothered to bind it to anything in the end and the board isn't programmable so that's that. It's just a dead key. I got the answer from Frostylove. Wish I'd known sooner, I spent so long trying to figure this out and wondering if it was something I'd ****ed up. Anyway, I don't have this board anymore so it's not my concern, but it's nice to have an answer, finally.