This is an arbitrary index used to keep track of the local-remote peer association, per its description, uniquely identifying multiple distinct associations between the same local and remote peer pair over time. It matters because a single local-remote peer pair can establish and tear down many Phase-1 tunnels over time, and this index lets an admin distinguish between multiple historical connections to the exact same peer rather than conflating them. Seeing several history entries share the same remote peer identity but different values here confirms that peer connected and disconnected multiple separate times rather than maintaining one continuous session.