This field reports the number of underrun packets transmitted on the interface, exposed here as cienaCesPmExtendedTxRxTotalStatsBinTxUnderRunPkts. On this Ciena CES port's PM data, an underrun means the transmit MAC ran out of data mid-frame because the internal bus or backplane feeding it couldn't keep up, which is a hardware/backplane-congestion symptom on this specific port rather than a cabling problem; and this is the running grand-total counter for the extended Tx/Rx PM group, tallied since the bin's own StartTime (in practice since the PM instance was created or last reset), so treat it as a lifetime tally rather than a per-interval number. For example, a healthy port would sit at 0 for underrun frames in this bin; even 30 is enough to warrant a look, since this class of counter isn't supposed to move under normal operation.