This counter tracks the number of good frames successfully transmitted out the router's Ethernet interface. As the Ethernet-layer analog of the Ethernet packet-total counter, this is the baseline 'everything's working' counter to compare against the various Tx error counters (collisions, DMA underrun, lost carrier sense); a healthy interface shows this counter dominating the total frame count by a wide margin. On a lightly-loaded management link this might climb by only a few frames per second, so a technician troubleshooting connectivity would expect this counter to be incrementing at all before looking for problems elsewhere.