Presentations for program output so that the objects presented will be acceptable to input functions. Suppose, for example, you present an object, such as 5 , as a TV channel. When a command that takes a TV channel as an argument is issued or when a presentation translation function is "looking for" such a thing, the system will make that object sensitive. Also, when a command that is looking for a different kind of object (such as a highway number), the object 5 is not sensitive, because that object represents a TV channel, not a highway number.
A presentation includes not only the displayed representation itself, but also the object presented and its presentation type. When a presentation is output to a CLIM window, the object and presentation type are "remembered"--that is, the object and type of the display at a particular set of window coordinates are recorded in the window's output history. Because this information remains available, previously presented objects are themselves available for input to functions for accepting objects.
An application can use the following operators to produce output that will be associated with a given Lisp object and declared to be of a specified presentation type. This output is saved in the window's output history as a presentation. Specifically, the presentation remembers the output that was performed (by saving the associated output record), the Lisp object associated with the output, and the presentation type specified at output time. The object can be any Lisp object.
Common Lisp Interface Manager 2.0 User's Guide - 20 Sep 2011