I think you're missing here a FQDN not associated with any of these, possibly not even existing according to DNS, but within "your" sphere of administration - that is, a FQDN which "you" could make exist or associate with one of the above, but haven't.
I don't think I would really like that usage, but it could be written into Case 1 if people want it so.
NAI realm names are required to be unique and the rights to use a given NAI realm for roaming purposes are obtained coincident with acquiring the rights to use a particular fully qualified domain name (FQDN). Those wishing to use an NAI realm name should first acquire the rights to use the corresponding FQDN. Using an NAI realm without ownership of the corresponding FQDN creates the possibility of conflict and therefore is to be discouraged.