You may have a product that ships in several boxes. For example a table/chair set may have a box for the table, then a box for each chair in the set. The Dimensional Shipping extension supports configuration of this via the Multiple Shipping Boxes tab on the product. We most often see customers using this with bundled products.
If a bundled product has been configured to use the multiple shipping boxes and it matches the criteria for the box then the bundled item will ship in that box.
In the above scenario you can see there are 2 products in the cart. They have been configured so that the bundled product ships in multiple boxes and the other product ships in a separate box.