This is a read-write switch that, per its description, enables the cisgIpsSgTunnelStop notification when set to true, firing whenever a control tunnel stops, again gated by the master cisgIpsSgNotifCntlAllNotifs switch. It's the natural counterpart to TunnelStart, giving an admin a real-time alert the moment a tunnel goes down rather than only discovering it later by noticing the active tunnel count dropped. A hub set up to page an admin on TunnelStop events would catch a critical partner's VPN dropping within moments rather than waiting for a scheduled health check to notice.