JTA transaction always begins & ends externally to the JTA entity manager.
Beans and Container, in case of bean-managed and container-managed transactions, respectively,
will begin and end the JTA transactions;
they make entities a part of their active transactions by enlisting the entity manager's persistence context.
The entity manager, therefore, only participates in an already active JTA transaction; it doesn't actually begin or commit a JTA transaction on its own.
Hence, there is no mechanism to specify the transactional behaviour of the entities. Their transactional behaviour is completely dependent on the caller