jboss-ejb3.xml is a custom deployment descriptor that can be placed in either ejb-jar or war archives. If it is placed in an ejb-jar then it must be placed in the META-INF folder, in a web archive it must be placed in the WEB-INF folder.
The contents of jboss-ejb3.xml are merged with the contents of ejb-jar.xml, with the jboss-ejb3.xml items taking precedence.
A simple example is shown below:
As you can see the format is largely similar to ejb-jar.xml, in fact they even use the same namespaces, however jboss-ejb3.xml adds some additional namespaces of its own to allow for configuring non-spec info. The format of the standard http://java.sun.com/xml/ns/javaee is well documented elsewhere, this document will cover the non-standard namespaces.
The root namespace http://www.jboss.com/xml/ns/javaee
The following namespaces can all be used in the <assembly-descriptor> element. They can be used to apply their configuration to a single bean, or to all beans in the deployment by using * as the ejb-name.
This allows you to mark EJB's as clustered. It is the deployment descriptor equivalent to org.jboss.ejb3.annotation.Clustered.
This allows you to set the security domain and the run-as principal for an EJB.
This allows you to set the resource adaptor for an MDB.
The IIOP namespace is where IIOP settings are configured. As there are quite a large number of options these are covered in the IIOP guide.
This allows you to select the pool that is used by the SLSB or MDB. Pools are defined in the server configuration (i.e. standalone.xml or domain.xml)
This allows you to select the cache that is used by the SFSB. Caches are defined in the server configuration (i.e. standalone.xml or domain.xml)