The window that opens another window is called parent window; the windows opened by the parent window are called child windows. All the child windows opened by the same parent window are sibling windows.
See The different types of windows for more details.
The operations performed on the menu options are the same regardless whether the menu belongs to a MDI window or not. However, additional features are available in the MDI windows:
Associating a menu option with a group
Associating a MDI child window with a group
Displaying the list of opened MDI windows
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