This field records how much the counter for physical-layer symbol errors — invalid line-code symbols the PHY detected on the wire changed during the specific collection interval this historical record captured — a frozen snapshot of that one interval's increase, not a live running total. For a network admin, on this interface a rising symbol-error count almost always traces back to a marginal cable, connector, or SFP rather than anything happening at the Ethernet MAC or above. For example, 1 symbol errors in the last interval on this specific FCoE path would already be worth a closer look, since this counter should normally sit at or near zero.