The Class Definition Pane displays the Smalltalk definition of the current class selected in the browser.

The definition is in fact just the Smalltalk message:

       #subclass:
       instanceVariableNames:
       classVariableNames:
       poolDictionaries:
       classInstanceVariableNames:

being sent to the superclass object. If you make changes here you can commit them by choosing Workspace/Accept or Ctrl+S.

Note that although the definitions pane is normally just used for browsing and changing class definitions, it is at the same time a workspace and the various workspace operations can also be used here.