Notification messages notify a control's parent window of actions that occur within a control. Sections 5.8.1 through 5.8.4 briefly describe the notification messages for each notification class.
Controls use the WM_COMMAND message to notify the parent window of actions that occur within the control. The wParam parameter of the WM_COMMAND message contains the control ID; the low-order word of the lParam parameter contains the control-window handle; and the high-order word of lParam contains the control notification code.