Convert Credit

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What is a convert credit? I belong to a website where I have 30 convert credits available and I don't know what they are or how to use them. I don't think there is info about them on the website- I think they assume that you know what one is. Is it unique to this site or do other places use convert credits? What do you do with them? What should I do with mine? Is there more than one use for them usually?