The ProductCode property is mandatory and it's added automatically when the instance is created. You can usebutton to generate a new GUID.
You can set additional properties for the selected instance. An example would be to set the ProductName property to a different value so the instances will be easily distinguished in the Control Panel->Add or Remove Programs applet. Use the [ New... ], [ Edit... ] and [ Remove ] buttons to select them.
- Edit Instance Property Dialog
The "Edit Instance Property" dialog in Advanced Installer.