RFC 5838 describes new support in OSPFv3 to move toward being a multiprotocol IGP adding support for address families. Since addressing information was moved away from Type 1 and Type 2 LSA in OSPFv3 to Type 8 and 9 allows OSPFs LSA format to be more flexible to carry different address formats. Also OSPF was augmented with an instance ID that allows multiple OSPFv3 process to communicate over the same link while remaining separate.
RFC 5838 split the instances to assign specific address families:
.The Options bitfield in the OSPFv3 Hellos , DD packets, and LSAs hase a new AF-bit defined. This bit is set if the sending router supports the address family extension and the specific instance is not an IPv6 unicast instance according to the table. Basically OSPFv3 routers will keep this bit cleared to indicate IPv6 unicast address families and will set it for all other instance Ids.
If the received an OSPFv3 packet from a neighbor without the AF-bit set, OSPFv3 will treat it as a separate plain unicast instance and will drop any subsequent OSPF packets from that neighbor and never form an adjacency.
Only OSPFv3 routers that support the same instance and AF features will ever form an adjacency.