Use shipping classes when the shipping charge for an item varies by product type. You do not need to create shipping classes if your shipping rules apply to all of your products.
For example, you may charge a flat rate of $2.50 for all CDs, $3.50 for all DVDs, and a rate based on the total price of the item for Electronics. To account for these variations, you will need to define different shipping classes. One class could be "DVDs," another "CDs," another "Electronics," and so on.
You should create as many shipping classes as necessary to account for all variations in your shipping calculations and should assign a class to every product in your feed to avoid having listings with no shipping information.
You can apply your shipping rules to particular product classes by creating a new rule or editing an existing rule and selecting the product class or classes to which you want the rule to apply. You may need to wait up to 24 hours for your feed update containing data in the shipping-class field to be processed before you can select shipping classes.