In graphical environments, every floating window has an implicit subwindow (and can have more than one if you create pop-up subwindows). This implicit subwindow is a region of the floating window. It cannot exist outside of the floating window, cannot cross the border of the floating window, and cannot be moved independent of the floating window. In essence, it is a viewing region in the active floating window.
Therefore, under graphical systems, the application always has two active windows: its active floating window and its implicit subwindow. When you create a new floating window, it starts off with a default subwindow that covers its entire interior (client area). You do not display directly to the floating window, but instead to its subwindow. A subwindow's coordinates (and thus the program's coordinates) are always relative to the floating window to which it belongs.
If you close a floating window, any subwindows associated with it are also destroyed.