This counts Forward Error Correction activity at the OTU layer: the number of bit errors the FEC decoder found and corrected within this bin, per the MIB's description. A raw FEC count on this OTU section isn't itself alarming -- FEC is designed to soak up routine noise -- but tracking how it trends bin over bin tells an admin whether the link's error floor is creeping up before it turns into uncorrected blocks or SES; this particular bin is a live counter that keeps accumulating until it is explicitly cleared rather than rolling over on a fixed 15-minute or 24-hour schedule, so a large value mostly reflects how long it has been since the last clear. A stable DWDM span might correct on the order of 4044722 bit errors in this bin purely from ordinary optical noise, with no visible impact on the client traffic riding on top of it.