Replace : Automatically closes any window(s) it intersects when it appears on the screen including popup other replace type windows. Overlay : Appears on top of currently displayed window(s) and can be larger than the window(s) it is overlaying. When an overlay window is closed, any window(s) that were hidden behind it will reappear. Clicking on any visible portion of a window behind an overlay window will bring that window to the foreground as the active window. Popup : Similar to an overlay window except, it always stays on top of all other open windows (even if another window is clicked). Popup windows usually require a response from the user in order to be removed.