Essential ActionScript 3.0: Static Variables and Static Methods
Source: O'Reilly Media
Static variables, by contrast, are variables associated with a class itself, rather than a particular instance of that class. Static variables are used to keep track of information that relates logically to an entire class, as opposed to information that varies from instance to instance. For example, a class representing a dialog box might use a static variable to specify the default size for new dialog box instances, or a class representing a car in a racing game might use a static variable to specify the maximum speed of all car instances.