This OID counts, in blocks, how many OTN frame blocks on this ODU (Optical Data Unit) layer of this 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 the ODU layer beneath an OTU4 wavelength carrying aggregated client traffic 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.