A prompt to add fallback details will appear upon connecting to a store. The algorithm uses such information to fall back on (at checkout or upon order sync) when products are saved without dimensions, weight, or categories.
Note: Ideally, you shouldn't need to use fallback information; it is the last resort in case products aren't updated on time and rates require generation at checkout. You should save products with the necessary information needed for accurate rate generation. More information on setting up products here: Adding and Editing Products.
To better understand the utility of product information at checkout, refer to the following article: How Live Shipping Rates Are Calculated via the Checkout Plugin.
We'll Cover:
Setup/Edit Fallback Details
Fallback details are configurable in Store Settings, under Products. 
Fallback Types
- Fallback Shipping Box: If all items in the order are missing dimensions, this will be used to determine the size of your shipment.
- Fallback Category: Your fallback category will be assigned any products with missing categories.
- Fallback Product Dimensions: Each product in the order without saved products will default to your fallback dimensions.
If your account already has saved Boxes, you can select your existing boxes while connecting to a store instead of creating a new one:
