What is the difference between the 1cup/2cup buttons for Breville Dual Boiler?

[yalag]

I read the manual and it says in both cases it run for 30s duration so what is the difference in the two buttons? It's not in volumetric mode so the shot is just controlled by duration isn't it?

Also, in manual mode it runs until you stop it so is that in 1cup "mode" or 2 cup "mode"?

Thanks.