The DDL is a dictionary of definitions which describes a language for specifying data definitions. DDL defines the data model that provides the foundation for the description of knowledge about an application domain.
The application knowledge is collected in a dictionary of definitions which describes the domain. DDL provides the framework on which this dictionary is organized by defining the levels of abstraction that are available to hold the data description. The DDL defines both the properties that may be associated with each level of abstraction and the relationships that may exist between levels. This DDL defines a relatively simple set of abstractions which include data blocks, categories, category groups, subcategories, and items.
Dictionaries use the abstractions provided by the DDL to describe the knowledge base of the application domain. The DDL presented here is expressed in the form of a dictionary that is based on its own underlying DDL. This self-consistent description not only provides a prototype for other application dictionaries, but it also provides a mechanism by which the consistency and relational integrity of the data model can be verified independent of an application domain.