Differences between using a notebook in the browser and in OneNote: OneNote Online vs. OneNote desktop app.
Microsoft OneNote Online lets you take notes and organize note pages in a web browser. For more advanced editing features, use OneNote Online ‘s Open in OneNote command. When you save the notebook in OneNote, it is saved on the website where you opened it in OneNote Online.
The notebook you open in OneNote Online is the same as the notebook you open in the OneNote desktop app, but some features work differently in the two environments.