Hi Nick, my cluster doesn't have one?? If nobody gets back to you on this you could meter it to see if any of tabs have a resistance with it? If not I would bet it's just a added ground/earth. If the tab is used for a ground, you should get resistance between that top tab and your grounding tab that fits into the cluster from the limiter.
Here a site that my help out in some stuff.
Jim :thumblef: