The phrase "optional fitment" means that something is not required or mandatory to be added or installed. It is up to the individual's preference or choice whether or not to include it. Full definition
An aero kit comprises optional fitment of carbon front spoiler lips and mirror covers, a carbon rear diffuser and carbon roof spoiler. (evo.co.uk)
The A6 range now gets Audi's Matrix LED lighting system as optional fitment. (indianautosblog.com)
Optional fitment of an optional four - wheel steer system, available on both four - and rear - wheel drive models, helps to increase agility, while optional self - parking premieres the world's first full self - parking system. (evo.co.uk)