optional

The history field trip is optional, so you only need to go if you are interested.

(Chinese) 这次历史实地考察是可选的，所以只有你有兴趣才需要去。

The new car comes with leather seats as an optional feature for an extra cost.

(Chinese) 这款新车配有真皮座椅作为可选配置，但需额外付费。