Windows for Layout Sets
A window is a logical unit within a layout set. As such, it does not have a physical position on a page. In contrast, a page window is a specific rectangular area on a page in which text is output. A page window is created when you assign a layout set page to a layout set window.
A window type is assigned to each window. A layout set can have many different types of windows, but it must contain one (and only one) window with the window type designation MAIN.