A bounding box (BBX) is a 2D plane that encompasses all of the supported route objects. The bounding box has two ports that define diagonally opposite corners of the box.
Bounding boxes are used by the CSD to determine the range (bounding box) of the items being supported. When creating an assembly that is intended to support more than one object, it is better practice to connect to the bounding box instead of the individual route objects. That way if another route object is added to the support later, the support automatically resizes because the bounding box automatically resizes to accommodate the new object.