8.4.1 Message-driven bean types
EJB V2.0 defined a single type of message-driven bean that enabled the asynchronous delivery of messages via the JMS.
EJB V2.1 expanded the definition of MDBs to provide support for messaging providers other than JMS providers by allowing an MDB to implement an interface other than javax.jms.MessageListener.
The type of message listener interface that an MDB implements determines its type. Therefore, an MDB that implements the javax.jms.MessageListener interface is a referred to as a JMS MDB.