I noticed that it is possible to define a data relationship between a Subscriber field and a data extension. Also see here "You can also include the subscriber record if you're relating a data extension to a subscriber attribute" The normal approach would be to link two data extensions instead.
So the question is: once I created the link (see screenshot ), how and where in the application would I filter the subscribers according to this relationship? It looks like I am not able to find it. It that is the case, I am wondering why the application allows you to define such link.