OneNote keeps backup copies of your notebooks. These backup copies of notebooks are maintained in the form of sections. The sections have the file extension .one. For example, the backup copy of a section called “Production” is called “Production.one.” To help identify when backup copies were made, OneNote lists the date they were made after the backup name.
After you open a backup copy of a section, OneNote places a tab on the Notebooks pane called Open Sections. With this tab, you can switch back and forth between backup sections you opened and your OneNote notebook. You can also copy notes, pages, and the section itself from the backup copy into a notebook.