ask FCK.ExecuteNamedCommand, help!

there's code like this in "fck_2.js"

FCK.ExecuteNamedCommand = function( commandName, commandParameter )
{ FCKUndo.SaveUndoStep() ;
if ( FCK.RedirectNamedCommands[ commandName ] != null )
FCK.ExecuteRedirectedNamedCommand( commandName, commandParameter ) ;
else
{ FCK.Focus() ;
FCK.EditorDocument.execCommand( commandName, false, commandParameter ) ;
FCK.Events.FireEvent( 'OnSelectionChange' ) ;
}
}

what kind of command that will give "FCK.RedirectNamedCommands[ commandName ]" = null ?
and what kind of command that will give not null value ? please anyone help me