This is a Counter64 accumulating the total number of Phase-1 control tunnels that have gone inactive over the device's lifetime, per its description, explicitly called out as a 'high capacity counter' because on a busy hub this number can grow very large over time. It gives an admin a sense of total tunnel churn history, useful when correlating a sudden jump in this counter with a known event like a WAN outage or maintenance window. A steadily climbing rate here that suddenly accelerates would suggest something is causing tunnels to terminate and re-establish more frequently than the historical baseline.