This OID counts, in blocks, how many OTN frame blocks on this OTU4 (100G optical transport) interface were received containing at least one bit error. This is a standard OTN performance-monitoring parameter, and a rising count indicates the optical path is experiencing bit errors at a rate significant enough to be visible at the block level, the basis for the errored-seconds and severely-errored-seconds counters derived from it. For example, a gradual increase in errored blocks on a 100G OTU4 wavelength spanning a metro DWDM link over several days, without yet crossing into severely-errored-second territory, would let an admin flag a slowly degrading span for proactive maintenance before it becomes service-affecting.