### Class Hierarchy

Object classes are hierarchically organized so that some classes may be located above some classes in the hierarchy and below others in the hierarchy. In any such system of subordinate classes, each next lower class inherits all the attributes prescribed for the class immediately above it, plus whatever attributes are defined peculiarly for it alone. If the hierarchy continues further down, cumulative collection of attributes continues to accumulate. If there were a class for every letter of the alphabet, starting at the highest level with A and continuing down to the lowest level with Z, and if each succeeding letter was a subclass of its predecessor, the Z class would possess all the attributes of all the other letters, as well as its own, while the A class would possess only the A class attributes.

XDS/XOM classes are seldom nested more than two or at most three layers. All inherited attributes are explicitly listed in the object descriptions that follow, so you do not have to worry about class hierarchies here. However, the complete descriptions of XDS/XOM objects in Part 4 of this guide rely on statements of class inheritance to fill out their attribute lists for the different classes. Refer to Part 4 for information about the classes of objects that can be returned by XDS calls in order to be able to handle those returned objects.

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