You can define presentation translators to make the user interface of your application more flexible. For example, suppose the input context is expecting a command. In this input context, all displayed commands are sensitive, so the user can point to one to execute it. However, suppose the user points to another kind of displayed object, such as a student. In the absence of a presentation translator, the student is not sensitive because only commands can be entered to this input context.
In the presence of a presentation translator that translates from students to commands, however, both students and commands would be sensitive. When the student is highlighted, the middle pointer button might execute the command show-transcript for that student.
Common Lisp Interface Manager 2.0 User's Guide - 3 Mar 2015