The prefix for caption filenames, in the form description-

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{language}

• description is a description of the video.
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  {language}

  is a literal value that Elastic Transcoder replaces with the two- or three-letter code for the language of the caption in the output file names.

If you don't include

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{language}

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{language}

For example, suppose you're transcoding into srt format. When you enter "Sydney-{language}-sunrise", and the language of the captions is English (en), the name of the first caption file will be Sydney-en-sunrise00000.srt.