This counts the number of BTUs, bus transfer units, the card's internal cell-bus transport unit, this card has sent across the chassis cell bus. Because BTUs are the internal backplane transport mechanism rather than external network traffic, this counter is most useful for diagnosing chassis-internal throughput or backplane contention issues rather than WAN-facing traffic problems. For example, an admin comparing cardOutBtus growth against cardOutPkts helps determine how much per-packet backplane overhead this card's traffic mix is generating.