The secretary shall by regulation establish a classification of goods and services for convenience of administration of this part, but not to limit or extend the applicant's or registrant's rights, and a single application for registration of a mark may include any or all goods upon which, or services with which, the mark is actually being used, indicating the appropriate class or classes of goods or services. When a single application includes goods or services which fall within multiple classes, the secretary may require payment of a fee for each class. To the extent practical, the classification of goods and services should conform to the classification adopted by the United States patent and trademark office.