The emacs man page for the Mx revert-buffer command says that it is bound to "su". What the hell? I know that Mx is Meta-x (usually alt-x), and I know that Cx is Control-x. But what is su?
The “s-” modifier is “super”, which is a window key on a standard PC keyboard.
This makes such shortcuts unusable in Windows AFAIK, but I have a few limitations on my Linux.
I don’t know which key matches the Apple keyboard.
On Mac OS X 10.6.8 (Snow Leopard) with Emacs: 23.3.1, su matches Cmd-u.
Summary: this is your Super key.
(Emacs is nothing if it doesn't match their name - apparently this is usually Meta)