What is the encircled question mark (AKA "Insert" or "fn") key?

In the spot above the delete forward (⌦) key, which is where the fn key would be in an Apple full size aluminum keyboard, and where Insert is in a non-Apple keyboard…

…the keyboard viewer shows me an encircled question mark. In hindsight it makes so much more sense, because whenever I would accidentally press Insert while one Apple Remote Desktop (ARP) the pointer would switch to a question mark icon and from that moment I know that would be it, I'd have to restart the server from the terminal (over SSH), and sometimes the client as well if I wanted to keep using it. Because there was no way of getting rid of the question mark pointer and while it is there you can't click on anything.

But what is it? What is it supposed to represent?

It struck me as if it were a forgotten/overlooked click-to-{get help on what's being clicked} pointer, as I remember back from the cat days, macOS or rather Mac OS X, had as a selling point you could ask for help in plain English. It all tied with the Apple Stores, the Genius Bar… the works. Apple no longer makes the effort, the OS comes without help if you're not connected to the Internet.

Why would they leave that in, it makes no sense. Of course, attention to detail has also been long gone… so it kinda circled back on making sense. :)

Anybody knows what does it mean? What do you call that key when it's not fn, nor Insert? Why does it get stuck when sent in ARP?

I guess that's it. Thanks!

I don't think it's relevant but I'm using various versions of macOS pre-11. 10.14 mostly– This is shown in all of them.