I came across this circuit which I think is to isolate a high voltage signal to a safe level for GPIO of a 3.3V micro controller.

My question is that how on earth this circuit works...There are 2 high voltage levels (28v and 12v) and BE_OPEN is a ~24V/0V signal...which I think comes from a push button.

What V16 does here? doesn't it block any currents that tries to go to pin #3 of the opto-coupler?

What about Z3 (zener diode)...what will happen if we remove it and just short to ground?