Subscribers can not only receive and extract Distributions, they can also pass on Distributions to other Subscribers. Subscribers that pass on Distributions are known as parent Subscribers.
Parent Subscribers do not need to be subscribed to the Distributions they are passing on. They simply receive a Distribution for passing it on to a subordinate Subscriber that has done two things:
To set up parent Subscribers for passing on Distributions:
Determine a Subscriber object (hereafter referred to as "child Subscriber") that cannot receive a certain Distribution because this child Subscriber is not contained in the Distributor's routing hierarchy (the Distributor owning this Distribution).
In that Subscriber object's properties, click the General tab > click Settings > in the Parent Subscriber field browse for a Subscriber object that is contained in the Distributor's routing hierarchy > click OK.
This establishes the Subscriber selected as a parent Subscriber. This distinction is not kept in the parent Subscriber's object properties, but only in the child Subscriber's.
Create a Channel object where only the child Subscriber is associated.
Create a Distribution > associate it with the child Subscriber's Channel.
Send this Distribution.
Because this Distribution is associated only with the Channel where the child Subscriber is subscribed, the parent Subscriber will not extract it, but only pass it on to the child Subscriber.
Because the parent Subscriber is in the routing hierarchy of the Distributor, it will have access to the Distribution for passing it on. Because the child Subscriber does not have any access to the Distributor, it needed the parent Subscriber to provide access to the Distribution.
Although you can establish a parent Subscriber for a child Subscriber, the child Subscriber can still be subscribed to a Channel where the parent Subscriber is subscribed. Both Subscribers can receive and extract that Channel's Distributions without the parent Subscriber passing it on to the child Subscriber, because the child can have access to that particular Distributor's routing hierarchy. The key is whether the Distributor owning the desired Distribution can send it to the child Subscriber without using a parent Subscriber.