Consider the following criteria when defining the database membership of a domain:
Current user database links operate only between databases within a single enterprise domain. Use of these links requires mutual trust between these databases and between the DBAs who administer them.
Accepted authentication types for enterprise users are defined at the domain level. Database membership in a domain should therefore be defined accordingly.
Note:
If one or more databases are intended to only support SSL-based certificate authentication, they cannot be combined in the same domain with password-authenticated databases.
Enterprise roles are defined at the domain level. To share an enterprise role across multiple databases, the databases must be members of the same domain.