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 18 feet (6 meters).
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.