So Google decided to add other meta tags for use with their Custom Search Engine (CSE) service:

  • Thumbnail: <meta name="thumbnail" content="http://example/foo.jpg" />
  • Department: <meta name="department" content="legal" />
  • Content audience: <meta name="audience" content="all" />
  • Document status: <meta name="doc_status" content="draft" />
  • Results sorting: <meta name="rating" content="5" />

https://support.google.com/customsearch/answer/1626955?hl=en
https://support.google.com/customsearch/answer/2595557?hl=en