Trademark Classes (Categories)
Even when a company’s mark is successfully placed in the Principal Register, the company does NOT have the exclusive right to use the mark throughout the entire business world. Rather, trademarks are awarded only for a specific class (category) of business. There is an international listing of trademark classes as defined by the International Classification of Goods and Services. Click here to view it. So as an example, technology giant Apple has been awarded a trademark for using the name Apple in certain industries (class 45, to be specific), and therefore no other company could legally call their technology company Apple. However, Apple has likely not been awarded a trademark to use their name in most other industries. So Apple could probably not be able to prevent someone else from starting a flooring company (Section 27) called Apple Rugs.
With that said, trademark law does have a term known as “Trademark Dilution” which can stretch between classes. Trademark Dilution occurs when a company outside the scope of a trademark holder’s business arena starts using a trademark holder’s name and thereby diluting the impact of the name. Dilution probably wouldn’t occur for Apple because “apple” is a common word. But if you started the “Google Rug Company” or the “Chevron Phone Company”, Google (the search engine) and Chevron (the oil company) might sue you for diluting their rather distinctive names. They would be likely to win based on how peculiar the names are and how famous the Google and Chevron names have become.