This read-write object selects the secondary (backup) clock source for the AUM card, chosen from the same options as the primary: internal oscillator, AUM inband clock, or external clock. It provides a fallback timing reference if the primary source configured in primaryMuxClockSource fails or degrades. An admin building a resilient timing setup would configure this to a different source type than the primary -- for example externalClock as primary and aumInbandClock as secondary -- so a single failure doesn't take down clocking entirely.