An electrical device like this requires a reverse leakage current of 100nA @ 21V.While the device is operating, the maximum voltage - Zener (Nom) is reduced to 28V.
ACZRM5255B-HF Features
reverse leakage current of 100nA @ 21V 100nA @ 21V is the maximum voltage (Tol)
ACZRM5255B-HF Applications
There are a lot of Comchip Technology ACZRM5255B-HF applications of zener single diodes.