Box Sizes
If you ship with the same box sizes on a regular basis and dimensional weight is a factor in your shipping rates, you'll want to add these defaults to save time as you prepare your shipments.
Once these are added and enabled, the "name" will appear in a drop down menu on the shipment creation page, saving you time in entering these dimensions.
Box Details
Not only can you specify the dimensions for each box size, you can also select the fixed weight for a specific box. You can also setup specific Freight Options for each box size, which will populate on Freight Shipments.