Frame-Window Styles (C++)
The frame windows you get with the framework are suitable for most programs, but you can gain additional flexibility by using the advanced functions PreCreateWindow and the MFC global function AfxRegisterWndClass. PreCreateWindow is a member function of CWnd.
If you apply the WS_HSCROLL and WS_VSCROLL styles to the main frame window, they are instead applied to the MDICLIENT window so users can scroll the MDICLIENT area.
If the window's FWS_ADDTOTITLE style bit is set (which it is by default), the view tells the frame window what title to display in the window's title bar based on the view's document name.
What do you want to know more about
Managing MDI child windows (MDICLIENT), the window within an MDI frame that contains the MDI child windows