A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
Hi Bahador,
Generally, .onetoc2 is a Microsoft OneNote index file that stores information about the order of note-taking sections. It is used to organize a collection of notes in a notebook. The .onetoc2 files only appear in OneNote notebook folders as Dave mentioned above.
If you would like to open a whole notebook, you can find the .onepkg file in the folder to view it normally in OneNote 2016.
Regards,
Yoga