Default propagation is always OK for simple cases where there are no nested database transactions. Consider this (where the SESSION and TX are not global XA resources, so their resources are orthogonal):

0   |  SESSION {
1   |    input;
2   |    RETRY {
3   |      TX {
3.1 |        database access;
|      }
|    }
|  }
		

Here there is a transactional message SESSION(0), but it doesn't participate in other transactions with PlatformTransactionManager, so doesn't propagate when TX(3) starts. There is no database access outside the RETRY(2) block. If TX(3) fails and then eventually succeeds on a retry, SESSION(0) can commit (it can do this independent of a TX block). This is similar to the vanilla "best-efforts-one-phase-commit" scenario - the worst that can happen is a duplicate message when the RETRY(2) succeeds and the SESSION(0) cannot commit, e.g. because the message system is unavailable.