Old keyboards back in the olden days had F1 to F24. Modern ones don't, but the computer still knows they exist and will handle them if you press them on such a keyboard.
Nothing nowadays interacts with these keys for the most part, which makes them great for streaming related automation hotkeys; if I get a headpat redeem, I can set streamer.bot / mixitup / etc to trigger an F13 press to turn it on, then F14 to turn it back off.
This is probably supported if i emulate the F13 keypress during the "assign key" dialog but this is kind of cumbersome to set up, so a manual selection of these would be great.