For a boundary event to catch an externally triggered event from a different track or process, the associated activity must be active. For example, if a catching boundary message intermediate event is added to a call activity, the call activity must be active before the boundary event can catch a message.

This is also true when debugging. Process Debug runs only one track at a time, meaning only one track is active at any given moment. Therefore, an activity in one track will never be active when another track is being debugged.

This situation can also conceivably occur in the run time if two or more processes are interacting, and one of the processes is not running.