As well as defining structural elements, an architecture defines the interactions between these structural elements. It is these interactions that provide the desired system behavior. Figure 2 shows a UML sequence diagram showing a number of interactions that, together, allow the system to support the creation of an order in an order processing system. Here we see five interactions. First, a Sales Clerk actor creates an order using an instance of the OrderEntry class. The OrderEntry instance gets customer details using an instance of the CustomerManagement class. The OrderEntry instance then uses an instance of the AccountManagement class to create the order, populate the order with order items, and then place the order.