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 DSL port's current, still-accumulating measurement period (not yet rolled into the interval history). 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 polling this counter mid-period and seeing it already climbing can flag a developing transmit problem before the period even finishes and gets archived into history.