The Union class is a special class that is used to enhance the expressiveness of the data model. Its goal is not the instantiation of corresponding data objects, but the usage of several data objects with different data classes as possible attribute values. Such a union can be used in one attribute or as base class in collections. For example, a SetClass that should contain elements of AggregateObjects as well as IntegerObjects. The base class that has to be specified in the set could be the common parent class of both data classes, i.e., the root data class. If this class is used, any data object of each data class could be an element of the collection, e.g., void objects. This problem is solved with a Union class that combines the Aggregate Class and the Integer Class into a new class that can be used as base class. It is not possible to instantiate this base class.