Is it mandatory to have discrimination between breakers in case of zone selective interlocking?
Zone selective interlocking (ZSI):
A ZSI terminal block may be used to interconnect a number of control units to provide total discrimination for short-time and earth-fault protection, without a delay before tripping .
So, Yes, it is mandatory that the breakers have proper Discrimination / Selectivity / Coordination for zone selective interlocking. Please refer the attached document for a detailed explanation on the subject.