This object reports which clock source (primary, secondary, or internal oscillator) the PXM card was using before its current selection took effect. An admin cares because comparing this to pxmCurrentClock reveals whether and when a clock failover recently occurred, which is valuable context when diagnosing timing-related transmission errors that started around the same time. For example, if pxmPreviousClock shows primary and pxmCurrentClock now shows intOscillator, a recent loss of the primary reference triggered a fallback to the less stable internal oscillator.