This writable value sets the errored-seconds (ES) count that, once exceeded within the current 15-minute SONET path interval, triggers an alarm, and the same threshold applies to both the near-end and far-end measurements. It gives an admin a tunable early-warning line for transient line degradation on this SONET path without waiting for the full day's tally. For instance, lowering this threshold on a critical backbone link makes the switch alarm sooner when a handful of errored seconds appear in a 15-minute window, before the fault worsens.