This setting makes it possible to preserve the server side session of the page containing the editor in the File Browser window. In this way it is possible to create custom File Browser implementations that may use sessions saved data.
FCKConfig.PreserveSessionOnFileBrowser = true ;
This is an IE specific issue. Other browsers will always use the main session. The drawback of enabling this setting is that popup blockers could block the editor to open the File Browser window and so the user will have to enable popups in the site.