Each application has one "main frame window," a desktop window that usually has the application name in its caption. Each document usually has one "document frame window." A document frame window contains at least one view, which presents the document's data.
For an SDI application, there is one frame window derived from class CFrameWnd. This window is both the main frame window and the document frame window. For an MDI application, the main frame window is derived from class CMDIFrameWnd, and the document frame windows, which are MDI child windows, are derived from class CMDIChildWnd.
These classes provide most of the frame-window functionality you need for your applications. Under normal circumstances, the default behavior and appearance they provide will suit your needs. If you need additional functionality, derive from these classes.