A "Trademark Class" is a category of goods and services as identified by the Trademark office. Goods and services can not be a part of the same class. For example, a company wants to trademark their logo for their t-shirt line. They will file an application for that particular class of goods. If that business also manufacturers and creates customized t-shirt, then that service will be in a different class. If the business wants to trademark both their goods (t-shirts) and services (customized manufacturing and creation of t-shirts), then they will have to file two separate trademark applications.