Add content is how you point Copilot at exactly the material you mean. Copilot is already grounded in what Microsoft Graph can reach, so a broad question searches everything you have access to. In Copilot Chat, the plus (+) icon in the message box opens a menu where you can add work content, upload files and images from your device, or attach cloud files from OneDrive, including files shared with you.
A narrowly scoped prompt is more accurate and safer: less noise to sift through, and less chance of surfacing an old, loosely shared file that should not have come up. It applies to the message you are writing, the narrowest of Copilot's context levels. An instruction you keep repeating across chats belongs in a Copilot Notebook or your personalization settings instead.