X (Google X)
X Development is an American semi-secret research and development facility and organization founded by Google in January 2010, which now operates as a subsidiary of Alphabet Inc.
is a special file containing pages that you want to be indexed by search engines and located in the website’s root directory. One file can contain up to 49 999 links, and its size should be less than 50 MB. If the website has 50,000 or more pages, several XML Sitemaps are created. There are different subtypes of XML Sitemap such as Image, Video, News, and Mobile. It should not be confused with other sitemap types (for example, TXT or HTML ones).
X-Robots-Tag HTTP header
The X-Robots-Tag can be used as an element of the HTTP header response for a given URL. Any directive that can be used in an robots meta tag can also be specified as an X-Robots-Tag .