This OID counts, in seconds, how many one-second intervals contained at least one errored block on this OTU4 (100G optical transport) interface. Errored seconds is a standard telecom performance metric that translates raw bit or block errors into a time-based measure operators use to judge whether a circuit meets its contracted quality-of-service or SLA thresholds. For example, if a 100G OTU4 wavelength spanning a metro DWDM link has accumulated far more errored seconds than its SLA allows for the month, that alone, even without an outright outage, can justify opening a maintenance ticket to investigate the optical path.