This read-only INTEGER identifies the type of a clock source without an associated ifIndex, with values none(1) when cwnInterfaceIndex is non-zero, internalOscillator(2) for a local onboard oscillator, and bitsClockE1(3) or bitsClockT1(4) for a Building Integrated Timing Supply clock on an E1 or T1 port respectively. An admin reads this specifically when cwnInterfaceIndex is zero, to determine whether the device's non-interface clock reference is its own internal oscillator or an external BITS feed. A device reporting internalOscillator(2) rather than a BITS clock would tell an admin it's relying on its own free-running clock rather than an external timing reference, which affects long-term accuracy.