Designs are interfaces components (or complete interfaces) that can be used to speed the process of interface creation. They provide both inspiration and raw materials for use in your work and can dramatically speed up the delivery of a high quality user experience.
Designs are brought into an interface by clicking either Get Started or Add Page at the top of the Pages Panel (Interface) and then selecting a page or all pages from the Design Selection Dialog.
When building a new interface it is easiest to click on Get Started and then select a design you would like to use from the Design Selection Dialog and start with a complete set of pages by clicking on the Copy Entire Design into Interface button. You can look through the Built in Designs and also study any Custom Designs you may have developed to select the best design to start with.
Once you bring the design across, you will likely want to delete pages, since designs often contain alternative versions of pages, such as landing pages, that have the same use. Then make adjustments to the appearance and connect up controls. An example of doing this is shown in Building a Simple Interface.
Using the Add Page button select the type of page you want to add from the appropriate design. If you have been building the Interface using a design you will probably want to select a page from that same design. It is, however, possible to take pages from any design, though you will likely need to make adjustments to have them fit well with the rest of your interface.
When you bring a page, or pages, into your Interface from a Design, it is likely that one or more Template pages will also be brought across. For the most part this is seamless, and you will not need to work directly with the templates. If you are mixing and matching pages from different designs, you may want to change the templates on some of the pages you bring in. It should be clear from appearance if this needs to happen (for example missing logos).
It is quite simple to create custom designs as described in Custom Designs. This is useful for setting up things like logos and colors in a way you might want to use for multiple interfaces.