This counter reports the total frames of unknown-unicast traffic on this port that exceeded the configured flood/storm-control threshold and were therefore dropped rather than forwarded, during everything accumulated since the last manual clear. Because this figure accumulates over the period since the last manual clear and is only zeroed when an operator issues a manual clear, a step-change in an otherwise-flat trend is the storm-control mechanism's record that a unknown-unicast flood happened on this port, caused by 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, and that record persists until someone deliberately clears it. For example, if this clearable total shows a jump of several thousand frames but its uncleared TotalStats-bin counterpart shows the same jump plus an older one this Stats bin no longer reflects, that difference tells you an operator cleared this counter sometime after the older unknown-unicast flood but before the more recent one.