Aggregate Roots

As we saw above, in the original creation of the Visitor entity by the Referrer, the visitor class wasn’t the aggregate root. Yet, in the user registration use case, the Visitor entity was the aggregate root. ... Aggregate roots aren’t a structural property of the domain model.

http://www.udidahan.com/2009/06/29/dont-create-aggregate-roots/ [so Dahan says aggregate roots  can vary by context?]