The CAPI (Common Application Programmer's Interface) is a library for implementing portable window-based application interfaces. It is a conceptually simple, CLOS-based model of interface elements and their interaction. It provides a standard set of these elements and their behaviors, as well as giving you the opportunity to define elements of your own.
The CAPI's model of window-based user interfaces is an abstraction of the concepts that are shared between all contemporary window systems, such that you do not need to consider the details of a particular system. These hidden details are taken care of by a back end library written for that system alone.
An advantage of making this abstraction is that each of the system-specific libraries can be highly specialized, concentrating on getting things right for that particular window system. Furthermore, because the implementation libraries and the CAPI model are completely separate, libraries can be written for new window systems without affecting either the CAPI model or the applications you have written with it.
The CAPI currently runs under X Window System with either GTK+ or Motif, Microsoft Windows and Mac OS X. Using CAPI with Motif is deprecated.
CAPI User Guide and Reference Manual (Unix version) - 3 Aug 2017