This object, included in an IS-IS adjacency change notification, indicates whether the reported adjacency protocol is IPv4 or IPv6. An administrator's monitoring system uses this to know which address family's adjacency the associated trap concerns, since a router may maintain separate IS-IS adjacencies for IPv4 and IPv6 with the same neighbor. Seeing ipv6(2) in a trap tells the admin the reported adjacency state change specifically concerns the IPv6 topology with that neighbor.