In the context of this chapter, an interface refers to any single window which is used in an application. Thus, an editor, an Open File dialog, or a confirmer containing an error message are all examples of interfaces. The GUI for a complete application is liable to comprise many interfaces. You can load as many different interfaces into the Interface Builder as you like, although you can only work on one interface at once. More formally, the class
is the superclass of all CAPI interface classes, which is the set of classes used to create elements for on-screen display. You can load any code which defines instances of this class and its subclasses into the Interface Builder.
Once you have invoked the Interface Builder, you can create new interfaces, or load any that have already been saved in a previous session. You must load or create at least one interface before you can proceed.
LispWorks IDE User Guide (Unix version) - 25 Nov 2011