This carries the tunnel index of whichever member LSP is currently the active one within a protection group at the moment the trap fires, which is exactly the piece of information a failover notification exists to convey. An admin uses this to confirm which physical/logical path the group's traffic is riding on right now, since the active member can change automatically without any operator action. For example, if this field flips from index 12 (the usual primary) to index 13 (the backup) in successive traps, that confirms an automatic protection switch just occurred.