Why might multiple Nodes be required to be reachable?
In some instances, for example, a service provider may wish to indicate that not only a particular User Node must be reachable, but also a Subscription Node. In other words, the device must not only possess a valid Link (or path of Links) to the User Node, but must also possess a path of Links to a particular Subscription Node, including a valid (e.g., unexpired) Link from a User Node to that Subscription Node.
