Englishfor English speakers
optional
Adjective
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If something is optional, you may choose it or not.
At this event, dress is optional.
clause
Noun
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A clause is a section in a contract or other legal document.
The author's contract contained a clause giving him creative control over the films.
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A clause is a group of words that usually includes a subject and a predicate. In English, the subject usually comes before the predicate.
The last clause in the sentence is hard to understand without knowing some background.