"Optional equipment" refers to additional items or features that you can choose to acquire or use, but you are not required to have them. Full definition
Other optional equipment includes front side airbags, fog lights, and anti-lock brakes. (automotive.com)
Customers can also avail an extensive list of optional equipment on the car including full LED headlights and a surround sound system, it added. (rediff.com)
18 - inch wheels come standard, but wheels up to 20 inches in diameter are available as optional equipment. (automobilemag.com)