Vortex Sound Research

I just got the moog model 15 app and am struggling to understand the moog 902 vca. I fed a sine wave into the vca and then to the trunk line. I then plugged the envelope generator into the control input of the vca. And that allowed me to play along with kB tracking. Then I tried to plug in another square wave into the control input but nothing happened.

My question is; why are there 3 inputs for the control input? What kind of signals can I plug into the control input jacks