Assembled Dimensions is a free addon that can be installed from Neto’s addon store.
Shipping dimensions are used in shipping cost calculations and the shipping consignment / label generation. Assembled dimensions are for descriptive purposes only and are used to inform customers about the assembled dimensions of a product.
The difference between the two is easily demonstrated with flat pack furniture. A shelving unit will ship unassembled, but the assembled dimensions are important for customers to decide if the product is the right fit for them.