Object class type

An object class can be one of three types, structural, abstract, and auxiliary.

Structural: Every entry must belong to at least one structural object class, which defines the base contents of the entry. This object class represents a real world object. Because all entries must belong to a structural object class, this type is the most common type of object class.
Abstract: This type is used as a superclass or template for other (structural) object classes. It defines a set of attributes that are common to a set of structural object classes. These object classes, if defined as subclasses of the abstract class, inherit the defined attributes. The attributes do not need to be defined for each of the subordinate object classes.
Auxiliary: This type indicates extra attributes that can be associated with an entry that belongs to a particular structural object class. Although an entry, can belong to only a single structural object class, it must belong to multiple auxiliary object classes.