When you create a new package, the default diagram of the package is defined according to the various parameters.
If you create a package using the Package tool from the Toolbox, the diagram is of the same type as the parent package or model.
If you create a package from the Browser, the diagram is of the same type as existing diagrams in the parent package or model, if these diagrams share the same type. If diagrams in the parent package or model are of different types, you are asked to select the type of diagram for the new sub-package.
If you create a package from the List of Packages, the diagram is of the same type as the active diagram.