This is part of the URL Meta plugin.
URL Meta Tag Indexing Plugin
Takes Meta Tags, injected alongside a URL (see NUTCH-655) and specified in the "urlmeta.tags" property, and inserts them into the document--which is then sent to the Indexer. If you specify these fields in the Nutch's schema (as well as the Indexer's), you can reasonably assume that they will be indexed.
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