Each design page, which is represented by a working file (.BE) is selected using the tabs at the top of the main window. Each page can contain any number of designs. Sometimes it is useful to break up your work into discreet areas that can be managed as a unit. Designs are the mechanism for this. For example, suppose you make a nice rose. Now you want a bouquet. You could make the rose as a single design, and then place it in multiple positions and sizes to complete your image.
If you have no designs on the page, one will be created for you when you either bring in artwork or begin to draw. However if you wish to switch to a new design to begin drawing a separate piece, you use the menu Create->Design->Begin New Design.