Besides a Main diagram, as available by default, you can create your own sub-diagrams. A sub-diagram is a new visual canvas you can use to create new objects, or add existing objects to. Sub-diagrams allow you to create sub-sections of the main diagram to clarify it's meaning or group items without having to deal with lots of items on the main diagram.
The upper part of the Diagram Explorer list the currently available diagrams. You can use the context menu in this area to create a new sub-diagram, another way is to the use New Diagram button and selecting New Sub-Diagram from the drop down. The diagram pages area also has a context menu that you can use.
The Diagram Explorer with sub-diagram selected
You can switch diagrams by using the diagram pages, or by selecting a diagram from the Diagram Explorer.
As you switch diagrams, you will see that the Diagram Editor retains the zoom factor and scrollbar positions of each diagram.