This counts Uncorrected FEC Blocks at the OTU layer: FEC-frame blocks that arrived with more errors than the FEC algorithm could fix, per the MIB's description. Unlike the pre/post-FEC BER estimates, which are modeled rates, this is a hard count of blocks on this specific OTU section that FEC gave up on and passed through with errors intact into the client payload -- any nonzero value here means real, uncorrected corruption reached the ODU/client layer during this bin, not just a close call; 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. On a healthy port this bin should read 0; even 2 uncorrected blocks would be treated far more seriously than a large FEC-corrected count, since it represents damage the correction scheme could not undo.