Z3 is a bidirectional 36 V transient suppressor, like two 35 V zeners in series back-to-back. It is there to limit clip any incoming voltage to no more than 36 V (actually more like 40 V. 36 V is the max voltage it is guaranteed not to clip.)

V16 prevents a possible +20 V signal ((36 V - P12V) attenuated by R139 and R140) from appearing across the optocoupler LED and damaging it.

The 240 Vac signal must be externally current limited or Z3 will fail.

Z3 is a bidirectional 36 V transient suppressor, like two 35 V zeners in series back-to-back. It is there to limit clip any incoming voltage to no more than 36 V (actually more like 40 V. 36 V is the max voltage it is guaranteed not to clip. (datasheet page 3))

V16 prevents a possible +20 V signal ((36 V - P12V) attenuated by R139 and R140) from appearing across the optocoupler LED and damaging it. Yes it blocks current, but its main job is to prevent reverse voltage.

The 240 Vac signal must be externally current limited or Z3 will fail.