B2 If something is optional, you can choose if you want to do it, pay it, buy it, etc.可选择的;非强制的
More examples
Seat-belts are included as standard but electric windows are an optional extra.
My phone came with 20 optional ringtones.
Do come to the party - fancy dress is optional.
There are several optional tours, for which there will be a small charge.
The basic model is quite cheap, but there are a lot of optional extras.
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