Are entity classes exclusive to one aggregate

As you rightly pointed out, Entity instances shouldn't be shared between aggregates, as one aggregate wouldn't be aware of changes to the entity made through another aggregate and couldn't enforce its invariants.

Entity classes could theoretically be shared between 2 aggregates, but, by the same reasoning, only if the set of entity instances in an Aggregate is disjoint from the other. This raises questions :

• Why would you want that in the first place ? If there are two big categories of instances of the same class, isn't this a sign that there are two semantically different concepts, which should each have their own class, or at least subclass ?

• How do you prevent an entity instance belonging to one aggregate from being added to the other, at runtime (bug), or at programming time (uneducated developer decision) ?

Value Objects escape these issues because they are usually immutable or treated as such -- you don't modify a VO, you modify its parent Entity so that it points to a whole new VO instance. Also, as Value Objects don't have an identity, it doesn't make much sense to say that the "same" VO is in two aggregates at the same time. You can thus safely reuse a VO type in different aggregate classes.