This reports which specific timing reference source the system is actually using right now for this protection group, as determined automatically by the clock-selection algorithm based on availability, configured priority, and received quality level, as opposed to cienaCesInputPGPreferredReference, which only records what the admin wanted. An admin monitoring this value in real time can detect a reference switchover the moment it happens, often before any alarm fires, by seeing this string change from the expected preferred source to a fallback source. For example, this value flipping from 'BITS-1' to 'GPS-2' during a maintenance window on the BITS feed confirms the failover worked as designed.