Seem, that Sybase will jump directly from 12 into 15 for Powerbuilder.
Frederick Koh explains one of the feature in version 15 for WindowType value called: mdidock! and mdidockhelp! for the window object. Like values mdi! and mdihelp!, respectively, they allow child windows (sheets) to be opened with the OpenSheet functions. But these sheets will behave differently: they can dock, float and be tabbed like the windows you see in our PB.NET IDE.