It specifies the virtual key code of the Control-menu key. (For a list of of standard virtual key codes, see Winuser.h)
When the context code is 0, WM_SYSCHAR can pass the WM_SYSCHAR message to the TranslateAccelerator Windows function, which will handle it as though it were a normal key message instead of a system character-key. This allows accelerator keys to be used with the active window even if the active window does not have the input focus.
For IBM Enhanced 101- and 102-key keyboards, enhanced keys are the right ALT and the right CTRL keys on the main section of the keyboard; the INS, DEL, HOME, END, PAGE UP, PAGE DOWN, and arrow keys in the clusters to the left of the numeric keypad; and the slash (/) and ENTER keys in the numeric keypad. Some other keyboards may support the extended-key bit in nFlags.
This member function is called by the framework to allow your application to handle a Windows message. The parameters passed to your function reflect the parameters received by the framework when the message was received. If you call the base-class implementation of this function, that implementation will use the parameters originally passed with the message and not the parameters you supply to the function.