What is the diagram tree used for?
The diagram tree is a bar in the navigation. It lists the diagrams of the current database, regardless of which package they are located in.
While the diagrams are nested in their relationship association within a package in the Explorer tree, they are displayed together under the diagram type names in the diagram tree. For example, if you create a Communication Diagram in the Explorer tree, it is placed under an interaction, while in the diagram tree it is placed under "UML2 Communication diagram".