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/perimeter zone is 15-20 feet (6 meters). A higher value set here will produce a lower results on the Dashboard (as Heat Gain and Daylight are more dispersed). A lower value will restrict the space more, resulting in higher results as the Heat Gain and Daylight are more concentrated.
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.