UML25 — Location p 152 Generalization Attributes: IsSubstitutable

current text: “isSubstitutable : Boolean [0..1] = true Indicates whether the specific Classifier can be used wherever the general Classifier can be used. If true, the execution traces of the specific Classifier shall be a superset of the execution traces of the general Classifier. If false, there is no such constraint on execution traces. If unset, the modeler has not stated whether there is such a constraint or not.”

[AC] I have always assumed that substitutability is a fundamental characteristic of generalizations, not an option. I may be wrong, of course, but I would like to see in the spec a discussion of this topic. If it is in the spec, I have not found it.

In the beta spec, isSubstitutable is not mentioned in the main semantics clause and should be.
Generalization, in object-oriented design, implies substitutability in some context. It is not always the case
that generalization implies substitutability in every possible context. The isSubstitutable property is intended
to indicate those cases