The United States follows an international classification system for trademarks. Under the system, applicants apply to register their marks within one or more classes that are based on the kinds of goods or services to which the marks apply. So, for example, batteries are in Class 9, automobiles are in Class 12, and shoes are in Class 25.
The classes make searching and grouping easier, but do not necessarily limit a trademark owner’s rights. For example, the USPTO may consider an existing registration covering goods in one class as conflicting with an application covering goods in another class if the goods are closely related. The full list of classes is shown on the USPTO website.