In a project of any length you’ll probably need to make notes along the way: things to look up or consider, little reminders, thoughts etc. Scrivener provides two ways to do this: Document Notes and Project Notes.
Both types of note are visible in the lower half of the Inspector pane. By default, when selecting an individual document in the Binder the Document Notes option is open, clearly distinguished by its yellow background – a bit like a sticky note, see? So if your note relates to this particular document, type it here. Great.
But things can get complicated.
more here - via Scrivener tip (Mac) – Project & Document Notes | Martin Sketchley.