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 most recent rolling 24-hour period ending at the moment of the poll. 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 pulling the rolling 24-hour underrun total at 3pm gets a true trailing day's figure, unlike the calendar-day 'Todays' counter which would only cover since midnight.