The older version of OpenSSO, which is Sun Java System Access Manager, does not utilize an embedded Configuration Data Store. As such, we usually utilize the same Sun Java System Directory Server to store both the configuration and users information. (unless, the users information are stored in Active Directory)
In OpenSSO, OpenDS is embedded to store Configuration information. It comes pre-installed with every OpenSSO bundle.
In fact, the recommended deployment approach is not to change this embedded data store.
Using the OpenSSO Enterprise embedded configuration data store can lower response time and ensure service availability when machine failure occurs.
What I like about this embedded data store is: if you scale by adding another node, there is nothing you need to do to ensure the configuration information are replicated and always in-sync. Replication is taken care of, transparently.