Each distinct piece of information in an application is represented as a variable in the constraint network. Each variable takes values from a given domain. At any point in time a subset of the domain of a variable constitutes its set of possible values. The possible values represent the system's view of the available choices for that variable, that is, those that are currently consistent with all the constraints.
In addition, each variable may have an assigned value. The assigned value of a variable can be set directly by the user or selected by the system from the possible values (see Section 4.3). If the user selects a value it remains as the assigned value unless it is changed again by the user or it becomes inconsistent with the constraints.
In the sample network of Figure 8 the variables are shown as dark rectangles and the assigned values as white rectangles next to them. The variables capture the relevant information for this task in the application domain, such as the DepartureDate, the Duration of the trip, the ParkingTotal (the total cost of parking for the duration of the trip), the TaxiFare, and the ModeToAirport. The DepartureAirport has an assigned value of LAX (Los Angeles International), which is assigned by the system since it is the closest airport to the user's address.