This counts cells queued for transmission on this port that were discarded due to ATM-layer errors, since the last reset. As the transmit counterpart to pportAtmLayerErroredReceivedCells, a rising count here on this specific port indicates this switch itself is generating cells that fail ATM-layer validation on egress, which would usually point to a local misconfiguration, like an invalid VPI/VCI mapping, rather than anything wrong with the far end. This counter climbing right after a new PVC was provisioned on this port would be worth checking against that PVC's VPI/VCI configuration for a mapping error.