Hello Russell, Bounding box is the containment for the element. If you would like to limit working space (ex for card size only), then you will need to set bounding box. Then your customers will not be able to add anything outside of the element.
You may specify, if it should be limited to any particular element, or you may set it by using setting like x, y, width and height.
Mode is a way of limited working area. Some will display only text - warning, another will hide text/image if it will cross bounding box border.