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// initialize DocFormat
DocFormat docFormat = new DocFormat(Common.mapSourceFilePath(path));
// get DocMetadata
DocMetadata metadata = docFormat.getDocumentProperties();
// check if metadata value is custom
if (metadata.containsKey("Affiliate") && !metadata.isBuiltIn("Affiliate"))
{
  // and remove it
  metadata.remove("Affiliate");
}
// and save changes
docFormat.save(Common.mapDestinationFilePath(outputPath));