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A Substitution is a relationship between two Classifiers, signifying that the substituting Classifier complies with the contract specified by the contract Classifier. This implies that instances of the substituting Classifier are runtime-substitutable, where instances of the contract Classifier are expected. In the example, the Class named ResizableWindow has a Substitution connector to the Class named Window, meaning that wherever you are asked for a window you can use a resizable window.
The Substitution relationship is a subtype of a Dependency relationship.