Currently the TP model assumes that the dispatch of a DISPATCHABLE_TASK (hand-off, system request, external request) is performed by the TP engine notifying the target. However, it is common for the subject to act as the notification, e.g. when a patient is given an appointment by an admin party, and expected to turn up at a clinic later. The work at the clinic only commences when (if) the patient appears. The subject is in this case part of the thread of control.