This should be a zener diode with temperature compensation. 12V zener diode has a positive temperature coefficient (zener diode greater than 6V has a positive temperature coefficient, and zener diode less than 6V has a negative temperature coefficient), and the regulated value increases with the increase of temperature. A common diode is connected in series as temperature compensation. Direct voltage drop of common diode has negative temperature coefficient. When the temperature increases, the forward pressure drop decreases (generally at -2mV/℃