This read-only counter tracks the number of times the serial port's transmit buffer ran out of data faster than the device could send it, an underrun error. A network engineer checks this metric to detect if the application is not feeding data to the port fast enough, which can cause gaps in transmission or dropped connections. For example, a value of 7 underruns over a week suggests minor delays, but 500 underruns per day would indicate the source application is too slow, the CPU is overloaded, or the serial link speed is mismatched.