1. In a computing environment, a method for specifying a hierarchy of user-defined classes of graphical objects comprising the steps of: graphically specifying a first class of graphical objects wherein an aspect of the first class of graphical objects is not fully specified; and graphically specifying at least one descendant class of graphical objects that is a subclass of the first class of graphical objects for the hierarchy of classes of graphical objects, wherein at least one of the first or the at least one descendant class of graphical objects can be instantiated in an executable graphical model; wherein the at least one descendant class of graphical objects supplies at least one aspect required for fully specifying aspects of the first class of graphical objects; incorporating an instance of the first class of graphical objects into an executable graphical model; instantiating the at least one descendant class of graphical objects; initiating execution of the executable graphical model; and dynamically specifying at least one of the first class of graphical objects or the at least one descendant class of graphical objects that is instantiated after initiating execution.