Panes are built from more general objects called sheets . Sheets are the fundamental "window-like" entities that specify the areas of the screen to be used for input and output interactions. Sheets consist of, in part, a region on the screen, a coordinate system, and optionally a parent and/or child sheets. For a complete discussion on sheets, refer to Chapter 18, Sheets. CLIM programmers will typically not need to deal with sheets directly, but instead will use the higher-level pane objects.
Common Lisp Interface Manager 2.0 User's Guide - 3 Mar 2015