The second trigger on the 990 is an addressible trigger - on each input's setup screen, there is a choice to have the trigger on or off. This means that (for instance) a front projector on the Video2 input could have a motorized projection screen that is commanded by trigger 2 to extend when that input is selected. The Model 950 has a main trigger (which behaves the same as the "trigger 1" on the 990) and a separate trigger for the second zone.