@ CONF iture
By B.P.
Not having used a derate climb, I can't speak with authority if it will cancel that or not. Otherwise it is basically a speed on elevator selection (like FLCH or OP CLB) but the speed is selected to green dot or VMo (depending on if you are otherwise in open climb or open descent)
I think the pilots wanted the ALT button more than they wanted the EXPED button.