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sponge:

1b: The light, fibrous, flexible, absorbent skeleton of certain of these organisms, used for bathing, cleaning, and other purposes (American Heritage Dictionary)

I would have written the above underlined phrase as ''which is used for...''. To me, the above phrase seems incorrect. Why would we start a sentence/phrase with a past participle? I know there are many phrases which have a past participle at the beginning. What is the reason for that?
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It's correct.
Comments  
Jackson6612sponge:

1b: The light, fibrous, flexible, absorbent skeleton of certain of these organisms, used for bathing, cleaning, and other purposes (American Heritage Dictionary)

I would have written the above underlined phrase as ''which is used for...''. To me, the above phrase seems incorrect. Why would we start a sentence/phrase with a past participle? I know there are many phrases which have a past participle at the beginning. What is the reason for that?

I don't quite get what seems incorrect to you. It is a reduced relative clause. Actually it is not a complete sentence. Relative clauses, whether reduced or not, are used to give information about a person or a thing.