Explain command routing ?

A command is a special type of message. Windows generates a command whenever user chooses a menu item, clicks a button or tells the system to do something. Windows send a WM_COMMAND message for menu choices, button clicks or WM_NOTIFY for list box selection. Commands and notifications are passed around by the operating system just like any other message until they get into the top of OnWndMsg() and MFC command routing starts.
Command messages have the first parameter, the resource id of the menu item that was chosen or the button that was clicked.
Command routing is a mechanism used by OnWndMsg to send commands to objects that can’t receive messages. Only objects of type CWnd can receive messages but all objects that inherit from CCmdTarget including CWnd and CDocument can receive commands and notifications.
How do commands and notifications get to the class though? The answer is by command routing. OnWndMsg calls CWnd::OnCommand() or CWnd::OnNotify() which in turn call OnCmdMsg() which is a virtual function, which means that different command targets have different implementations. The implementation of a frame window sends the commands to its views and documents. This is how commands get handled by member functions of an object which is not a window and hence cant catch messages.