For modifier keys you can use shift, ctrl, alt, option, meta, command & mod.

Other special keys are backspace, tab, enter, return, capslock, esc, escape, space, pageup, pagedown, end, home, left, up, right, down, ins & del.

Any other key you should be able to reference by name like a, /, $, *, =.

You can assign many keys to a single operation, simply by separating them by ,.