How exactly means partecipate in a given transaction in Spring working with JPA?

I am pretty new in Spring and in JPA and I am studying for Spring Core certification and I have some doubts about this question found on the study material:

Are you able to participate in a given transaction in Spring while working with JPA?

I think that the answer is no because I know that JPA provides configuration options so Spring can manage transactions and the EntityManager but I am absolutly not sure to have correctly understand the question. What exactly means with participate in a given transaction.

At the beginning I interpreted as to interact with an existing transaction and this is impossible because a transaction is atomic for definition.

But them into the documentation I found this informations:

To transparently participate in Spring-driven transactions:

• 
  

  Simply use one of Spring’s FactoryBeans for building the EntityManagerFactory

  
  


• 
  

  Inject an EntityManager reference with @PersistenceContext

  
  

Define a transaction manager: JpaTransactionManager

and show this example:

public class JpaOrderRepository implements OrderRepository {

    private EntityManager entityManager;

    @PersistenceContext
    public void setEntityManager (EntityManager entityManager) {
        this. entityManager = entityManager;
    }

    public Order findById(long orderId) {
        return entityManager.find(Order.class, orderId);
    }
}

So is it related to my original question? How?

What exatly do the @PersistenceContext annotation and what is the exact role of the EntityManager object?

Tnx