This field is the average, as frozen at the moment this historical snapshot was taken, of the per-interval delta values for physical-layer symbol errors — invalid line-code symbols the PHY detected on the wire, taken across the retained collection history for this stats object. Admins watch this to separate a genuinely sustained problem from one noisy interval — if DeltaAvg for symbol on this interface is climbing, on this interface a rising symbol-error count almost always traces back to a marginal cable, rather than a one-off spike. For example, an average of 2 symbol errors per interval sustained over many polling cycles points at a chronic issue rather than a one-time glitch.