the fact of choosing to take part in an activity, arrangement, etc. rather than being forced to take part主動選擇加入
It is an opt-in rather than an opt-out system, which means that permission must be sought on a case-by-case basis.這是一個主動選擇加入而非主動選擇退出的系統,這就意味着準許令必須酌情逐個考慮。