Links as attributes for links
When you create a relationship, thus linking a field, you may choose atteibutes for it. Yet it is not possible to choose another link as that attribute.
Let me explain myself- you may want to link an entry to a different library field, but that just apply according to some other third library. So it is mandatory to have slme kind of nested linking. A links to B, with attribute C, where A, B and C are all linked fields from different libraries, not static values.
Imagine the list of possible attributes is so big that it'd rather be made as a separate library and linked, not just items from a dropdown list field.