optional /ˈɑːpʃənl̟/ adjective Learner's definition of OPTIONAL
: available as a choice but not required可供选择的;非强制的 -
Jackets are required at the restaurant, but ties are optional.这家饭店要求必须穿夹克式外套,但领带不是必需的。
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Many optional features are available on this car.这款汽车提供了很多可选择的功能。
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Registration is optional, not mandatory.注册是可选的,不是强制的。