This is the cumulative loss-of-synchronization events counter: counts how many times the receiver picked up an optical signal but could not lock onto the FC bit clock and word boundary needed to decode it. On one specific FC port's current error-statistics object, this is the lifetime tally, so this is the classic symptom of a marginal transceiver, a dirty fiber connector, or a speed mismatch with the far end, and it is distinct from an outright signal loss. For example, a port that autonegotiated down to 8Gbps against a 16Gbps-capable SFP logs repeated sync losses until an admin forces the correct speed.