Dummy is a blank sample with the final paper and binding. It also includes cover surface lamination result. That means, the dummy is used final cover paper and interior paper, and the binding method is exactly same as the final product.
According to the dummy, you may check the book thickness, the book size, the book cover paper, interior paper and cover surface with lamination. The dummy just doesn’t include printing.
To check the printing color, we can do a set of hard proof. So, from the dummy and hard proof, you will know what’s your final book looks like. You will have an idea of your final book.