This read-write threshold defines the low water mark for the transmit holding queue: once frame discarding has started due to congestion, it continues until the queue depth drops back down to this level, at which point discarding stops. An admin sets this in conjunction with lportSevereThreshold and lportMildThreshold, the high-water discard triggers, to create hysteresis, preventing the discard mechanism from rapidly flapping on and off at a single threshold. For example, if discarding starts at lportSevereThreshold's high mark and the queue then drains, it must fall all the way to lportXmitDiscardLow before the switch resumes accepting all frames again.