Attachments go with a specific conversation. Use them for one-off PDFs, images, or memos you are working through in that chat.
Good practice
Some agencies maintain a document library (SOPs, manuals, templates) that the assistant can draw from across many chats. Those files are usually curated by admins.
Good practice
If the UI offers suggested documents, treat suggestions as shortcuts, not endorsements. Pick what actually applies to your question.