The frontage bonus rewards buildings that sit away from other buildings and expose more of their perimeter. It can add up to 166% to your allowable area — potentially cutting the required number of buildings in half. The surface shows the full bonus landscape. The sliders show your specific position on it.
Purpose: the frontage bonus is the most underused area bonus in IBC. Most setbacks easily achieve 20ft of open space. The question is how much perimeter you're willing to expose.