An electrical device like this requires a reverse leakage current of 50nA @ 21V.While the device is operating, the maximum voltage - Zener (Nom) is reduced to 30V.For operation, diode reaches 900mV @ 10mA at the maximum voltage - Forward (Vf).
BZX84W-C30X Features
reverse leakage current of 50nA @ 21V 50nA @ 21V is the maximum voltage (Tol)
BZX84W-C30X Applications
There are a lot of Nexperia USA Inc. BZX84W-C30X applications of zener single diodes.