This OID counts, in blocks, how many background block errors, error blocks not occurring during a severely errored second, have been detected on the receive side of this STM-64 (10G SDH) interface. Background block errors capture the steady, lower-level error background of the circuit, separate from the bursty errors counted during severely errored seconds, giving an admin a view of the circuit's baseline signal quality. For example, a slowly rising background-block-error count on a legacy STM-64 SDH trunk in a metro transport ring, even with zero severely errored seconds, could be the earliest sign of a marginal optical budget on that span.