A Notebook narrows what Copilot can see, on purpose. Standard Copilot Chat grounds every prompt in your mail, files, chats and more. Inside a Notebook that switches off: Copilot draws only on the references you added, such as Word, Excel or PDF files, Copilot Pages, OneNote pages and specific web URLs. A notebook draws on up to 300 files for an answer.
That makes it the right tool when nothing else may influence the answer, like a contract comparison or a compliance review. A notebook can also produce summaries, drafts and audio overviews, a spoken summary of its material. It resembles a Claude project, but a Project adds knowledge on top of Claude's normal behaviour, while a Notebook takes reach away.