The menu handle is the handle to the popup menu that the MDI client will add items to representing each window that is created, allowing the user to select the window they want to activate from the menu, we'll add functionality shortly to handle this case. In this example it's the 3rd popup (index 2) since I've added Edit and Window to the menu after File.
ccs.idFirstChild is a number to use as the first ID for the items the Client adds to the Window menu... you want this to be easily distinguishable from your own menu identifiers so you can handle your menu commands and pass the Window menu commands to DefFrameProc() for processing. In the example I specify an identifier defined as 50000, high enough that I know none of my menu command id's will be above it.
Now to get this menu to work properly we need to add some special handling to our WM_COMMAND handler: