This is the tunnel-group index bundled into a trap, identifying which protection group (a set of tunnels that back each other up, e.g. primary plus one or more standby LSPs) the notification concerns, rather than a single standalone tunnel. An admin needs this because a group-level event -- like a failover -- affects which tunnel is currently active, information a single tunnel index alone would not convey. For example, a trap referencing tunnel group index 8 tells the admin to check the whole protection group's active-member state, not just one LSP.