Double wall boxes are made of two attached walls, for heavy duty stacking and rigidity.
if you are stacking pallets several layers of boxes, for example, 6-8 layers, and your contents per box weight 30 lbs plus, it is a good idea to use double wall boxes to avoid the edges crushing or the boxes bursting.
Even in Double wall, there are different board tests, depending on the application. If you are shipping out apparel or garments, then 200 lb double wall boxes will be sufficient in most cases. If you are shipping out super heavy duty items, like heavy but parts, you may want to go 350# DW.
If you don't see the size you need, contact us and we'll quote you a custom box size particular to you product.