Interface classes (subclasses of interface) are (mainly) used to define top level windows and the components inside them. Normally, each kind of a window in an application is specified by a different interface class. Complex dialogs are also typically presented using an interface class.
An interface class can also be used to create a component made of several elements. This is especially useful when these elements need to interact, because the syntax of define-interface makes it easier to refer to elements in the interface. To distinguish between this usage and the more typical case where an interface instance corresponds to a window, the latter case is referred to as a "top level interface" (also "top level window"). The parent of a top level interface is a screen (or document-container inside MDI on Microsoft Windows) rather than another pane.
An interface class is defined by the macro define-interface (normally, cl:defclass inheriting from an interface class works too). define-interface is an extension of cl:defclass with additional options for specifying display elements. After an interface class is defined it can be used to display a window or a dialog by calling display or display-dialog on an instance of it. For example:
(capi:define-interface my-interface ()
()
(:panes (my-display-pane capi:display-pane :text "Some text"))
(:default-initargs :title "My title"))
(capi:display (make-instance 'my-interface))
CAPI User Guide and Reference Manual (Windows version) - 3 Aug 2017