Substitution

A substitution is a relationship between two Classifiers. In these cases, the substituting Classifier complies with the contract specified by the contract classifier. Instances of the substituting Classifier are runtime substitutable where instances of the contract classifier are expected. 
Specify the Substitution element in the Substitution Specification window.

You can see which elements the Substitution is drawn between in the Substitution dialog, Source and Target boxes.

You can format the Substitution symbol properties in the Symbol Properties dialog.