Substitution block can be used to show that if a property holds for one object, then it holds for an equivalent object (object substitution).
Similarly, if a property holds for some object, then an equivalent property will also hold for the same object (property substitution).
Or there can be a double substitution, where both the object and property change.
The nature of the ‘equivalence’ will vary with the object and property, and will need to be defined and justified.