This read-only PerfTotalCount reports the count of transmit underruns, where the DSL port's transmitter ran out of data to send before the next bit was due, scoped to the running total for the current calendar day so far. Operationally, underruns point to the CPE or line card failing to keep the transmit pipeline fed, which can corrupt outgoing frames and hurt throughput. For example, an admin fielding a call mid-afternoon checks today's running underrun total to see how the problem has trended since midnight.