The output protocol is concerned with the appearance of displayed output on the window associated with a sheet. The sheet output protocol is responsible for providing a means of doing output to a sheet, and for delivering repaint requests to the sheet's client.
Sheets either participate fully in the output protocol or are mute for output. If any functions in the output protocol are called on a sheet that is mute for output, the sheet-is-mute-for-output error will be signaled.
Common Lisp Interface Manager 2.0 User's Guide - 3 Mar 2015