This read-only CoriantTypesOpticalPower value, defaulting to -23.0, is the loss-of-signal power threshold applied to the protection port. It tells an operator the point at which the backup path itself would be declared dead, which is critical to know because a failure of the protection port while traffic depends on it means there is no further fallback. For example, if protection-port power falls to -26 dBm against this -23.0 threshold while working has already failed, the group would report a total loss of service rather than a successful failover.