Explanation/Reference:
What Persistent Managed Classes Does This Persistence Unit Include?
You can specify the persistent managed classes associated with a persistence unit by using one or more of the following:
* The annotated managed persistence classes contained in the root of the persistence unit.
The root of the persistence unit is the JAR file or directory, whose META-INF directory contains the persistence.xml file. To exclude managed persistence classes, add an <exclude-unlisted-classes> element to the persistence unit.
* <mapping-file> element: specifies one or more object-relational mapping XML files (orm.xml files).
* <jar-file> element: specifies one or more JAR files that will be searched for classes.
* <class> element: specifies an explicit list of classes.
Reference: Oracle Containers for J2EE Enterprise JavaBeans Developer's Guide, Configuring the persistence.xml File