The OneNote notebook for each Dynamics 365 record will be automatically created and be available in all Dynamics 365 apps. ...

The OneNote notebook for each Dynamics 365 record will be automatically created and be available in all Dynamics 365 apps. To access a notebook, a user navigates to the form and chooses the OneNote tab (in the activity wall). Note that section groups in a OneNote notebook aren't supported.
This doesn't replace the current Notes feature, but gives users another way to access notes stored in OneNote.