As WebLogic does not use this element would that imply that the J2EE spec speficies that it is optional for vendors to support this element? How would I find this information out?
When you use the distributable element and run an App on a single Tomcat server (i.e. not in a cluster) it throws an exception if you attempt to add a non-serializable object to the session. So this is a good guard when developing an App on a single Tomcat server that will eventually be deployed to a cluster i.e. it will ensure that you aren't adding an non-serializable objects to the session.
I'm guessing that since WebLogic does not use this element, that adding it will not cause WebLogic to throw an execption if you add a non-serializable object to the session. Does WebLogic have some similiar way of marking the App as being suitable for running in a distributed environment, causing it throw an exception as per my example even when the App is running on a single WebLogic Server.