"Optional xenon" refers to a feature or upgrade that is available for something, like a car's headlights, but is not automatically included. Full definition
Infiniti L - shaped headlights and fog lights with optional xenon headlights with Adaptive Front lighting System (AFS), thin LED taillights (japanesesportcars.com)
At the front of the vehicle there is a distinctive stone gray single - frame grille and flat headlights with optional xenon plus headlights and LED daytime running lights. (topspeed.com)
The Estate model has optional xenon lights, a tire pressure monitoring system and the hill hold system. (thesupercars.org)