Hello Chris,
Good question, this can be confusing. Originally, OSPFv3 was created only for IPv6:
https://tools.ietf.org/html/rfc5340
Later, OSPFv3 was updated to support address-families:
https://trac.tools.ietf.org/html/rfc5838
This means you can run it for IPv6 and IPv4 (and some other address families like multicast).
The “ipv6 ospf” commands are the “old” commands and the “ospfv3” commands are the newer commands.
If you use “ipv6 ospf” on an interface and later use “ospfv3” then you’ll see that the “ipv6 ospf” command will disappear.
“ipv6 router ospf” is the old way to start an IPv6 OSPF process. “router ospfv3” is the new way to start OSPFv3 with address family support.
Hope this helps!
Rene