Historically camel-jpa would lookup from exchange header / property an EntityManager to use instead of from its own component. This was needed by jbpm and other 3rd party frameworks that integrated with Camel.
However this causes pain for normal users, where you just want to use the jpa component out of the box with the JPA EM it has been configured with. And if you add other jpa components to integrate with other databases, then they should out of the box use their own EM and so on. This is how JMS, Kafka and other components work.
So to allow the old behavior, we add a boolean option on the jpa component to turn on the old style. And then leave it off for new style onwards.