This read-only INTEGER identifies the origin of the clock: internal(1) a free-running local oscillator, interface(2) a line interface's recovered clock, or external(3) an outside reference. An administrator cares about this because the timing quality feeding circuit-emulation and voice traffic across the whole node depends on which type of source is in use. For example, a node configured to use interface(2) as its clock source is deriving timing from a specific incoming line rather than its own internal oscillator, making that line's stability critical to overall network timing.