This reports the index of the previously-active control tunnel that this history entry corresponds to, per its description, and must correspond to an expired IKE tunnel. It's the join key an admin uses to correlate a row in the history table back to whatever other records referenced that same tunnel by its original index number. Cross-referencing this index against archived logs from when the tunnel was still active lets an admin reconstruct the tunnel's full lifecycle from establishment through its eventual termination.