This PerfCounter64 counts Errored Seconds encountered by a SONET/SDH Section (the regenerator-to-regenerator segment) during the history bin. Section-layer ES is the earliest layer at which framing and byte-level errors are visible, so an admin checks it to localize a fault to the physical span closest to the interface rather than to a higher SONET layer. If ocnStmHistorySectionESs climbs on one port while the Line-layer ESs on the same port stays low, that mismatch points the admin toward a section-level framing issue, such as a section overhead mismatch with the neighboring regenerator.