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Union
A value of union specified for the property string indicates that the association end is derived by the union of the subsets with the name specified. This mechanism allows the classifier at the association end marked with union to specify its instances based on the union of its subsets. This is an application of the mathematical concept of union of sets and subsets. This implies that the collection is a set and cannot be a bag since a union results in a set, which means it cannot have duplicate elements.
Explanation
The value union is used when a property is intended to be specialized. Specifying the value of union at the ancestor level means that the instances that will be part of the collection at the association end are defined to be the union of the instances defined at a descendant level, which subsets the association end. It is a useful property when a modeler cannot specify at a higher level of inheritance how an association will be used.
