The zener diode is the reference and more than that the voltage divider (t2+r10k) will stop t1.
The transistor circuit's Supply and Zener compensates the voltage drop through ( the transistor and the two right resistors lowering) for low voltage applications.
In case the components can't handle high voltages:
the transistor circuit's equivalent resistance is about 577 Ohms, in case you need to add a voltage splitting resistor in series with the circuit to support high voltages to control TRIACS that may control AC. And you may handle the resistor's wattage..
