A referential attribute (aka foreign key) is an attribute that refers to another attribute which is an identifier (aka key) in another class. Shlaer-Mellor supports the concept of combining two referential attributes that must always have the same value. This is a powerful way to constrain the data set represented in your model.
This concept can be extended to non-referential attributes. One such technique is termed collapsed referential. A collapsed referential combines a referential with another attribute that is not a referential (base attribute).
The elegant model here depicts using this approach to propagate the identifier of a tree into every node of the tree.