This counter reports the total bytes of unknown-unicast traffic on this port that exceeded the configured flood/storm-control threshold and were therefore dropped rather than forwarded, during a completed prior day (for example the 24 hours of August 5th). A nonzero or rising value accumulated over that completed day is the storm-control mechanism actively protecting the rest of the network from a unknown-unicast flood, so it directly signals a wave of frames destined for a MAC address that aged out of the forwarding table, or a MAC-flooding probe against the switch's CAM table is happening on this specific port, not a general congestion issue elsewhere in the network. For example, a wave of frames destined for a MAC address that aged out of the forwarding table, or a MAC-flooding probe against the switch's CAM table could push this counter from near zero up to several megabytes within that completed day before dropping back to zero once the source is disconnected -- exactly the transient event this counter exists to catch.