You're right. You use this file if you're using CMP Entity Beans. This is the extension needed by Persistent manager to handle your CMP beans.
More the EJB spec goes and fewer should be things you need to say in that file.
However, here states container specificities that make it most attractive than others.
I'm not an EJB expert so maybe someone can correct me if I'm wrong.(BTW, it's the rule here.)
So can I use JBossCMP as a third-party persistence manager for app servers such as SunONE?