To try and explain better,
If a program has a main window, and other child windows, it is MDI.
e.g. mIRC is MDI, Notepad isn't
The use of .mdi here is a little wrong (I think) but means "is the child window a <normal> child" (confined within mIRC's main window) or not.
Hope it helps a little, I'm not good at explaining.
