All data types are either a restriction (based on) or an aggregation of other data types.
The basis for all GDTs are the CDTs provided by the UN/CEFACT CCTS.
Directly built upon the CDTs are the basic GDTs. A basic GDT represents a simple business-related business subject matter and its structure is limited to the maximum structure as defined at the CDT it is built upon. Therefore the structure of a GDT built upon a CDT is always a restriction on the CDT's structure. The restriction may be on the length, the value ranges, or the omission of Supplementary Components.
A basic GDT may base on another basic GDT. In this case the business-related subject matter of the basic GDT must be a semantic refinement of the basic GDT it bases on. Its structure must be a restriction of the one defined by the underlying GDT.
More complex business-related subject matters are represented by aggregated GDTs. They are described by elements. Their elements may have a simple or again a complex business semantic. Hence, they are typed by CDTs or GDT which may either be basic or aggregated. By this very complex business-related subject matters can be represented and structured by aggregated GDTs with elements that are typed in turn by aggregated GDTs.
An aggregated GDT may base on another aggregated GDT. In this case the business-related subject matter of the aggregated GDT must be a semantic refinement of the aggregated GDT it bases on. Its structure must be a restriction of the one defined by the base GDT. The restriction may include the omission of elements and attributes or strengthening of their cardinalities.
GDTs can be classified according to criteria like representation, functional area, corresponding business-related subject matters.
The classification of GDTs is currently in preparation.