The diagram displays the extensions in the VEC Modell necessary to support a mapping to external sources. The extension consists of two classes ExternalMappingSpecification and ExternalMapping. An ExternalMappingSpecification defines a mapping for a single DocumentVersion specified by the association mappedDocument. The referenced DocumentVersion itself identifies by its attributes the external document which should be mapped (e.g. an SVG-File, a requirements-document).
The mapping of the individual elements in the VEC is defined with the class ExternalMapping. It points to an ExtendableElement (which can be almost any VEC-Element) as mappedElement. The attribute externalReference contains the unique key of the same element in the context of the mapped DocumentVersion.
The reason for defining the ExternalMapping elements within a separate Specification is that the ExternalMappingSpecification can be placed in the VEC within its own DocumentVersion. The creation of mapping does not require a modification and thus a versioning of the mapped content in the VEC.