This identifies the decapsulation (decap) tunnel group -- the set of tunnels handling the unwrapping side of traffic -- that the notification is reporting on, mirroring the encap group index but for the opposite traffic direction. Because MPLS/GMPLS services often use separate encap and decap tunnel groups for asymmetric or protected paths, this index lets the admin tell whether an event affects inbound (decap) or outbound (encap) handling. A trap with decap group index 12 means the receive-side unwrapping path for that service changed state, not the transmit side.