In ForeUI, a mockup (or wireframe, or prototype) project is called "plot", which can be saved as a ".4ui" file. You can open multiple plot files in ForeUI and switch the current editing plot by clicking its tab.
A plot can contain multiple pages. Pages can be organized with tree structure (like site map).
Each page can contain many elements. You can drag as many elements as you need from the element categories (on the left) to the plot editing area.
The figure below shows the relationship between plot, page and element: