Level 2 Structures (L2S) are composite structures that consist of multiple assets and/or other Level 2 structures and are demarcated by a labeled box. Top L2Ss usually represent real world objects and are by default considered as first class citizens. Nested L2Ss can add additional structure but are by default not regarded as first class citizens. An L2S can be explicitly marked as first class citizen. First class citizens are drawn with a solid outline, non first class citizens are drawn with a dotted outline.
Within a business scenario, typical L2S are Business Objects and Business Documents:
Within each L2S one designated asset or nested L2S acts as identifying item. We demarcate each identifying item with a bold outline. A Level 2 Structure is unambiguously identified by its identifying item and belongs to the same namespace.
The following constraints apply to identifying items:
To comply to the second constraint may require to reverse the direction of an arc. This transformation of arc reversal represents a change of interpretation: An asset that was originally superior to the identifying asset becomes now a subaltern of the identifying asset. Semantically, this is a reification.
Note: Reverted arcs thus support the definition of structures with hierarchical navigation. Starting at an identifying asset it becomes possible to reach any asset within the same L2S. This sits well with model implementations in object-oriented or XML environments. In relational environments, however, the non-reverted original arc is better suited to the navigation methods found there. Tools therefore may want to carry the characteristics of the original arc with a reverted arc in order to better support relational target. environments.
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