The transport security is specific to the transport protocol mapping.
When using a broker-based middleware (see 18.104.22.168.2), confidentiality and integrity can be ensured with the transport security between Publishers and the Broker as well as Subscribers and the Broker. The Broker level security in addition requires all Publishers and Subscribers to have credentials that grant them access to a Broker resource.
Transport security may be hop-by-hop security with some risk of man-in-the-middle attacks. It also requires trusting the Broker since the Broker can read the messages. Combining transport security with message security reduces this risk.