When you first start a diagram, it is empty.
You can then use a predefined template or just draw the diagram by drag and drop.
To draw by drag and drop, Display the Palette
To use a predefined type Right Click
You can choose from various types of diagram.
When you create a diagram from empty using the Right Click menu, it also sets the diagram class and type - which otherwise you have to set in the diagram details form
You can also insert a predefined template from the Collections tab in the Palette , even when the diagram is not empty. This does not affect the page class