This counts Errored Seconds at the ODU (Optical Data Unit -- the inner G.709 container that carries the actual client payload): the number of one-second intervals in the bin during which an error was detected on the client payload container carried inside this OTN wrapper. ES at the ODU layer tells an admin how many seconds of the bin saw trouble specifically in the mapped client signal (e.g. the 10GbE or SONET payload riding inside this ODU), which is what actually matters to the service running over this circuit even if the underlying OTU section looks clean; because this bin is a closed, archived 24-hour interval, admins typically read it the next day to judge whether the previous day as a whole met SLA rather than to catch a fault as it happens. A typical reading for this bin might be 29 errored seconds, which an admin would compare against the bin's total duration to get a rough availability percentage for this specific interval.