This is the writable alarm threshold, defaulting to 20, compared against the path's current 15-minute Errored Seconds counter. Lowering this threshold makes the path more sensitive to reporting minor error bursts sooner (useful on a circuit carrying sensitive traffic), while raising it tolerates more noise before alarming, which an admin would do on a known-noisy long-haul span to reduce alarm fatigue. With the default of 20, a current-interval ESs count of 25 would trip this threshold and, combined with the statistical alarm severity setting, raise an alarm of the configured severity.