Not all state machines participate equally in the dynamics of a domain. In planning a system of more than a few state machines, we need to delegate responsibilities to the various state machines.
Some state machines tell others what to do and get an immediate response. For example, the order tells the credit card charge to make a charge, and the order receives an immediate response indicating success or failure. Similarly, the shipment makes a request to the delivery company and receives a response first to assign a tracking number and then to confirm delivery. These examples of hierarchical control can be seen in Figure 14.8.