This read-write threshold sets the transmit-queue depth at which the port enters severe congestion and begins discarding both red, definitely non-conformant, and amber, marked as discard-eligible, packets, a more aggressive discard policy than the mild threshold's behavior. An admin lowers this to protect the port more proactively at the cost of dropping marked traffic sooner, or raises it to tolerate deeper queues before triggering the harsher discard policy. For example, if lportSevereThreshold is set to 200 packets, the queue crossing that depth causes both red and amber-marked frames, typically from PVCs exceeding their committed rate, to start being dropped, not just the excess red ones.