A task listener must be added to a process definition within a user task. Note that the listener is a child of the BPMN 2.0 extensionElements element and is in the activiti namespace since a task listener is an Activiti BPMN 2.0 extension.
The following diagram shows an XML fragment from a process definition that contains Content Services-specific task listener.
Listeners are described in detail in the Activiti user guide.