Use case of three pattern doubt

Hi.

In the test, you say that in the Contact list of users is generally not represented in an hierarchical structure and therefore it is not ideal to use a Tree Pattern.

Why not? I mean, there is really a hierarchy when we have contacts and there is a real tree when we could also create a graph of it to compute relations.

Could you explain it to me? Thanks.

Contact lists of users

This is generally not represented in an hierarchical structure, therefore it is not ideal to use a Tree Pattern in this use case.

Please provide an example of such a hierarchy?