How does retention get applied to a OneNote Notebook

@jonathan- We have been testing this extensively over the last month and found that your SME might not have been entirely accurate, unless our tenant is different than others.  We found that the the OneNote notebook acts as a folder for .one and .onetoc2 files.  .one files are the sections you see in your notebook and the .onetoc2 files are a table of contents that holds the colors of sections and the order of sections and pages in the Notebook.  

The retention policy runs based on the last modified date of the .one and .onetoc2 files, not the notebook date you see.  For the last month I have been running 3 tests on a notebook:

1. I create a new section every day and then never edit that section again.
2. In a different section, I create a new page every day.
3. In a third section, I edit the same page every day

In #1 above the entire section is deleted after the retention period has been met.  In #2 and #3 the sections and all of their pages remain in tact as long as they don't meet the retention period before anything within that section is edited again.  

As far as the .onetoc2 file is concerned, I've only see that deleted when the last section is deleted from the notebook.  If that file is deleted and a user wants to restore the section(s) that were deleted with it, it should also be restored, otherwise I seem to get some funky behavior with section names and orders.  

In all cases the notebook (which acts as a folder) remains indefinitely, which is what your SME might have seen. But if you open that notebook the sections may have been deleted.  

Final note: you can see the .one and onetoc2 files via powershell.  We wrote a powershell script to review all of the files in OneDrive so we know the extent of the deletion that will occur when we turn the policy on.