This PerfCounter64 counts the Failure Count encountered by a SONET/SDH Line in the history bin, i.e. how many times a line-level failure condition (such as loss of signal) was declared during that interval. An admin reviews Line FCs to see how many discrete outage events, rather than just cumulative error-seconds, a line suffered in a bin, which matters for distinguishing one long outage from several short flaps. If ocnStmHistoryLineFCs reads 3 for a bin where the line's total UAS is high, that tells the admin the line dropped and recovered three separate times rather than staying down continuously.