When there are no rates set-up for a product group in a shipping zone, and that product is in an order to that zone, no rate will appear from the app at checkout.
This is true for when an order contains only that product group, and also is true if that product is part of a blended rate.
Product Group A has rates configured in Domestic zone
Product group B does not have rates configured in Domestic zone.
When Group A is the only group present in an order shipped to Domestic, rates will show.
When Group B is the only group present in an order shipped to Domestic, rates will not show
When Group A and B are present in order shipped to Domestic, rates will not show.
Using Rate Failure for Zone Restriction
You can use this rate failure functionality to restrict products from being ordered to certain zones.
In the above example, products from Group B are restricted from the Domestic zone.
They are restricted because when there are no shipping rates at checkout, a customer cannot proceed to the next step of the checkout to complete their purchase.
Editing the Rate Failure Message at checkout
Using the Shopify Checkout languages feature you can customize the static text that shows when no rates are returned.
Go to Shopify Admin > Settings > Checkout, then scroll and find the section Checkout Language
Click Manage checkout language button
Either scroll down and look for, or do a browser search for ‘no shipping method available’ to find the error text.
Edit that text to something more descriptive. Indicate that some products in the order are not available to their destination, and advise them to return to the cart page.
You can add some liquid code to your cart page to highlight or give notice on the products that have zone restrictions.