Core Offset allows you to change the depth of the perimeter from the Façade (i.e. the area that is directly exposed to solar load and natural daylight). A typical core offset is 20 feet (6 meters). A higher value set here will produce a lower results on the Dashboard.
The settings given in the The Entire Building Selected window are applied to the entire building. If you wish to make changes to a specific Façade or Surface only, you can do this by selecting individual Surfaces or Facades which allow you to focus on specific areas.