Per its DESCRIPTION, this Counter64 counts errored blocks that occurred outside of any Severely Errored Second, isolating the steady low-level bit-error background from error bursts. Because this is a lifetime counter that only resets on an explicit clear, a reading here means a low, steady trickle of bit errors (as opposed to a burst) has been accumulating on this OC-n (SONET) port, accumulated over the port's entire service life, not merely since the last reboot. For example, 4 background block errors with almost no severely errored seconds describes a slightly noisy but stable link, not one that is actively failing.