I have the same question. Can some one give us pointers to find out more about Service Archive (SAR)?
The SAR file (http://wiki.jboss.org/wiki/Wiki.jsp?page=ServiceArchive) contains the jBPM engine, config etc. So this should be 'deployed' before the WAR file. On other systems, like tomcat, the jbpm engine etc, could be in the WAR file, but that is a packaging issue. In the first case, the engine can be used by multiple war files. In the latter, it can't.