This message causes a multiple document interface (MDI) client window to maximize an MDI child window. When a child window is maximized, Windows resizes it to make its client area fill the client window. Windows places the child window's System menu in the frame's menu bar so that the user can restore or close the child window and adds the title of the child window to the frame window title.
wParam
Contains the window identifier of the child window.
lParam
This parameter is not used.
If an MDI client window receives any message that changes the activation of its child windows, and if the currently active MDI child window is maximized, Windows restores the currently active child and maximizes the newly activated child.