JPA....DUvida referente ao comando MErge

public boolean alterarCadastro(PessoaFisica pessoaFisica)   
    {   
           
        factory = Persistence.createEntityManagerFactory("banco");   
        manager = factory.createEntityManager();   
           
        EntityTransaction tx = manager.getTransaction();   
        tx.begin();   
        try   
        {   
            pessoaFisica = manager.merge(pessoaFisica);   
               
            manager.persist(pessoaFisica);   
            manager.flush();   
            manager.clear();   
            tx.commit();   
           
        }   
        catch (RuntimeException e)   
        {   
            tx.rollback();   
            JOptionPane.showMessageDialog(null, e, "Erro!", 0);   
               
            manager.close();   
            factory.close();   
               
            return false;   
        }   
        manager.close();   
        factory.close();   
        return true;   
    }