This counts Section-layer Loss of Signal seconds on this SONET/SDH port: seconds during the bin in which no usable optical signal was being received at the Section layer, per the MIB's description. LOS on this port is as close to 'the fiber went dark' as a PM counter gets -- any nonzero value in this specific bin means the receiver had literally no signal to lock onto for that many seconds, which is categorically more serious than any error-rate counter and usually correlates with a fiber cut, unplugged patch, or failed transmitter somewhere on this path; because this bin is a closed, archived 24-hour interval, admins typically read it the next day to judge whether the previous day as a whole met SLA rather than to catch a fault as it happens. Even 2 seconds of LOS recorded in this bin would normally coincide with a hard-down alarm on this port, whereas a healthy port should show 0 for every such bin.