The Pine view is that messages are stored in folders, and
    folders are stored in folder collections. Folder collections
    can be physically located on the local system, or on any remote
    system with an IMAP server. Regardless of what system a folder
    collection is physically located on, a Pine user sees it as just
    another folder collection: a Pine user can read messages in any
    of their folder collections and can save (move) messages between
    different folder collections.