Multiple-Document Interface (MDI) Applications
Multiple-document interface (MDI) applications enable you to display multiple documents at the same time, with each document displayed in its own window. MDI applications often have a Window menu item with submenus for switching between windows or documents.
There are some behavior differences between MDI forms and single-document interface (SDI) windows in Windows Forms. The Opacity property does not affect the appearance of MDI child forms. Additionally, the CenterToParent method does not affect the behavior of MDI child forms.