The Physics Builder include different types of links, such as Component Link, Property Link, Feature Link, and others. These links make it possible to define common building blocks and the use them in the physics interfaces that you create. In the Settings window for these link node, None is the default option in the Link list for new and reset links. If you use the link in a physics interface that you create, you must replace None with an actual link to a node under Building Blocks, for example; otherwise, an error occurs when running the physics interface.