Sorry for the simple question but what exactly is an boundary event and the behavior of it in JBPM ? is it an event that will be executed if a sub-process with a boundary event declared is started ? and what is it normally used for ? I'm having a bit of trouble finding reference to this in the user guide.
A boundary event is an event attached to a boundary (a wrapper for one or more activities). So for example you can attach a listener to an event to that boundary that can cancel all the activities inside that boundary. If you want to look the generic and starndard definition you can go to the BPMN2 specification where those events are defined.