This read-only InterfaceIndexOrZero identifies the ifIndex of the ATM Virtual Interface (ifType atmVirtual(149)) associated with this clock source entry, or is zero if the entry isn't tied to an ifIndex, in which case its type is identified instead by cwnOtherClockSource. An admin uses this to link a network clock source entry back to a specific physical or virtual interface on the device when the clock source is interface-based rather than internal. A value of zero here would tell the admin to look at cwnOtherClockSource instead to understand what type of non-interface clock source this entry represents.