188.8.131.52.4.1 Class Categories
There are four categories of classes:
Structural classes: Structural classes are the classes that can have instances in the directory.
Abstract classes: Abstract classes are templates that are used to derive new classes. Abstract classes cannot be instantiated in the directory.
Auxiliary classes: Auxiliary classes contain a list of attributes. Adding the auxiliary class to the definition of a structural or abstract class adds the auxiliary class's attributes to the definition. An auxiliary class cannot be instantiated by itself in the directory.
88 classes: 88 classes do not fall into any of the preceding categories. An 88 class can be used as an abstract class, a structural class, or an auxiliary class.
Structural class, abstract class, and auxiliary class are defined in [X501] section 8.3. 88 class corresponds to the definition of object classes described in [X501] section 8.3.4. 88 class is included for compatibility with this older standard and is not intended to be used in new schema extensions.