Explanation/Reference:
Application-Managed Entity Managers
With an application-managed entity manager the persistence context is not propagated to application components, and the lifecycle of EntityManager instances is managed by the application.
Application-managed entity managers are used when applications need to access a persistence context that is not propagated with the JTA transaction across EntityManager instances in a particular persistence unit. In this case, each EntityManager creates a new, isolated persistence context. The EntityManager and its associated persistence context are created and destroyed explicitly by the application.
Reference: The Java EE 6 Tutorial, Managing Entities
http://docs.oracle.com/javaee/6/tutorial/doc/bnbqw.html#bnbra