This object identifies the source feeding the Secondary Mux Clock on the PXM card, the backup timing reference used if the primary source specified in pxmPrimaryMuxClockSource fails or is unavailable. An admin cares because this is the fallback timing path — verifying it's configured to a genuinely independent source (not the same feed as primary) is essential for real clock redundancy on the chassis. For example, if both pxmPrimaryMuxClockSource and this object point to the same external reference, a single failure of that reference would take down both clock paths simultaneously.