11. The system of claim 10 , further comprising means for re-using a pre-defined data type definition associated with an entity to define target data types describing relationships of the entity.
10. A system for referencing a pre-defined data type definition associated with an entity to define target data types describing relationships of the entity, said system comprising: a memory area for storing a markup language schema corresponding to a model representing objects in a system; and a processor configured to execute computer-executable instructions for: identifying a data type definition from the markup language schema, said data type definition corresponding to a reference entity in the model; defining one or more elements describing one or more entities expected to be associated with the reference entity in the model; identifying predefined entity relationships for use in associating the expected entities with the reference entity, said expected entities having a target data type definition associated therewith, said reference entity having a reference data type definition associated therewith, said expected entities not being required entities for the target data type definition; associating the defined elements and the defined relationships with a tag; inserting the tag with the associated elements and relationships into an instance of the target data type definition in the markup language schema corresponding to the model; and locating the instance of the target data type definition in the reference data type definition to relate the expected entities with the reference entity in the model.