The System menu, identified by a box at the left end of the title bar, is a pop-up menu that contains the system commands. (The System menu is sometimes referred to as the Control menu.) The system commands are commands that can be selected by the user to direct Windows to carry out actions that affect the window, such as moving and closing it.
To create a window with a System menu or Close box, the application must specify both the WS_SYSMENU and WS_CAPTION window styles when the window is created.