To view an item on the right side column in Split View, hold down Ctrl + click (on Windows) or Cmd + click (on Mac) on a note or email.
Tip: This allows you to open two messages at the same time.
If you open something in Split View, it will remain there until closed or replaced by another item opened in Split View.
In the navigation side bar on the left, you can view Recent and Pinned Notes.