This counts frames not transmitted, or that had to be retransmitted, because the internal transmit FIFO ran dry due to insufficient PCI bus bandwidth, a DMA underrun. This points specifically at an internal hardware or bus bandwidth limitation on the IP card itself rather than anything wrong with the external network, so a rising count here alongside otherwise normal Ethernet cabling and switch behavior would suggest the card itself is being pushed beyond what its internal bus can sustain. A card handling an unusually high volume of small packets that pushes this counter up steadily might need its overall throughput reduced or offloaded, since the bottleneck here is internal PCI bandwidth rather than the Ethernet wire itself.