Graphical operations performed on panes must ultimately be carried out by the window system of the underlying host computer. This is accomplished primarily via communication with an underlying object called a medium . A medium understands how to implement CLIM graphics operations, such as draw-line , by calling the underlying host window system's graphics functions. A medium also contains default drawing options, such as foreground and background colors, clipping region, transformations, line thickness, and fonts. There are different medium classes to support different windowing systems; thus, there is one medium class for the X Window System and a different one for the Macintosh Common Lisp environment.
This host-specific behavior is kept in a separate medium so that the pane classes themselves will be host-independent. Thus, when you build a new pane class, you do not have to build one version with X graphics mixed in, another one for the Mac, and so forth.
CLIM application programmers will not usually deal with mediums directly. In most cases, panes will automatically be allocated a medium upon creation, and output directed to the pane will be appropriately forwarded to the medium. In situations where efficiency is a concern, you may choose to send graphical output directly to the underlying medium. There are also situations, particularly when a pane has infrequent output, when you may wish to have many "light-weight" panes that share a medium.
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017