A ClassFactory instance is a "factory object" which Flex uses to generate instances of another class, each with identical properties. The ClassFactory class implements the IFactory interface. When you create objects that has a property of type IFactory like itemRenderer property of alist control or the itemEditor property of a DataGrid control. When we change the item renderer at run time then we have to need classFactory in flex.
var myObject :ClassFactory = new ClassFactory(myItemRenderer);