The SDI element is the top-level element for a manual or other document, mainly aimed at online distribution. A MAP is the same thing. If you create a new MAP document, it will get the sdi element as it's top level element. Each element inside is linked with the drop and release technic in a special so called MAP Editor.
Inside an SDI (Single Document Interface), you organize your chapters with folders and ie elements, in the same way as you use map-sections in a map-book.