What principle are the Save and Undo functions based on?
When saving and undoing, all changes in all databases are included.
As a consequence, it is possible to undo or redo all unsaved changes that were made in Explorer, in a diagram, or on a Properties tab.
The best way to illustrate this is by creating and undoing a diagram. When you create a diagram, you can undo the individual steps, which means that the diagram is completely removed from the UML top-level element again. You can then restore all steps, which recreates the diagram in the Explorer tree and opens it in the diagram window where it is gradually rebuilt.
When you close diagrams you have changed, you are not prompted to save them. As a result, changes within a diagram can still be undone after closing the diagram. The save prompt is displayed when you perform an action that requires saving or when you close the program.