The interface that would next be used for the system clock source if the zhoneSystemCurrentClockSource becomes inactive, it's elgibility is changed,is being used for the clock source. A value of 0 is returned if there are no other available interfaces in the zhoneSystemClockTable that have a zhoneSystemAvailabilityStatus of active even the system clock table is not empty.
Sources for this description
Multiple manufacturers; see the original MIB MIB copies (LibreNMS/Observium Community)