Hi, given the size of the BPMN2 spec, jBPM focuses on what OMG defines as the "Common Executable Conformance" subset of BPMN2. This subset afaik does not include Transaction SubProcesses. For the list of currently supported elements by the runtime, see chapter 6: http://docs.jboss.org/jbpm/v5.2/userguide/ch06.html. As far as defining your own transaction boundaries, see http://docs.jboss.org/jbpm/v5.2/userguide/ch07.html#d0e2914.
Hope this helps.