As you already know, every page contains a tree-like group hierarchy. It was also already mentioned (in Basic actions) that each page has its own undo history and that the edit actions performed on a page are independent on actions performed on other pages. You can undo and redo actions in a page without any effect on other pages' undo histories, regardless of the global historical order of edit actions. These undo histories are called “local”.
To store edit actions not connected to contents of any page, VRR has the “global” undo history. It can be opened in the Undo History window (The Undo History Window), too. It contains creating, deleting, selection changes of documents and pages (not the pages' contents), changes of global settings etc.
Note: When you create a page, edit its contents and then undo a global action, you delete the page. But the contents of the page stay untouched; if you redo the global action, you restore the page together with all its graphic objects. The same holds for deleting pages, and creating and deleting documents works similarly.