When you are added to a shared OneNote notebook, you can opt in to email notifications whenever someone else makes changes, rather than having to reopen the notebook repeatedly to check. This feature has been around for a long time and, unlike a lot of Office web features, it is still available in essentially the same form today.
Managing your notifications
The cleanest way to control this today is directly at onenote.com/sharenotificationsettings. That page lists every shared notebook you have been subscribed to, and lets you turn notifications on or off per notebook, or unsubscribe from all of them in one action if you no longer want any of it.
How to tell what is actually new
Inside a notebook itself, unread content is shown in bold. A Notebook name, Section name, or Page title in bold means something under it has changed since you last looked. Unread pages specifically are also highlighted in green, which makes it easy to scan a busy shared notebook and jump straight to what actually changed rather than re-reading everything.
Turning notifications off for a specific notebook
If one particular shared notebook is generating more email than it is worth, whether that is a very active project notebook or one you no longer need to track closely, unsubscribing from just that notebook on the settings page above stops the emails without affecting notifications from any other shared notebook.
For more Microsoft 365 tips like this, browse the rest of our blog, or if your team relies heavily on shared OneNote notebooks and wants help getting more out of Microsoft 365 day to day, our coaching and enablement service is a good place to start.