This Counter64 totals the notification payloads received across all currently and previously active Phase-1 control tunnels, per its description - IKE notification payloads carry status and error information between peers, distinct from the SA-negotiation payloads themselves. A high or rapidly rising rate of notification payloads can indicate peers frequently reporting errors, deletions, or other status changes rather than a healthy quiet steady-state. An admin noticing this counter climbing faster than the tunnel count would grow would investigate what kind of notifications are driving the increase.