If you need to set up a Sitemap page for your website, it is advisable to have it in XML format rather than a plain TXT file.
The XML format of a Sitemap index file is very similar to the XML format of a Sitemap file. The Sitemap index file uses the following XML tags:
<sitemapindex>- the parent tag surrounds the file.
<sitemap>- the parent tag for each Sitemap listed in the file (a child of
<loc>- the location of the sitemap (a child of
<lastmod>- the last modified date of the sitemap (optional)
See the Sitemap Protocol for more information.