The button 
allows you to modify the broadcasting mode in a cartstack.
In mode , when a cart is broadcasted, if you click on another cartr to select it as the next item to play, it is displayed in blue and then a fader start, a click or the use of the DDO 110 will launch it.
In 
mode, when a cart is broadcasted, if you click on another cart, the broadcasted cart will be stopped and the selected one will be launched.