A sitemap index is a file that points to several individual sitemaps instead of containing all the URLs at once. It is used on large websites, where a single sitemap runs into the limit on the number of URLs or the file size. The practical benefit, however, is greater than just getting around the limit. If you split the sitemaps by content type – categories, products, articles, landing pages – you will see the indexing rate for each group separately in the search engine's reports, which immediately shows where the problem is. Include in the sitemaps only URLs that are meant to be indexed, so no redirects, error pages or excluded URLs. State the date of the last substantial change, not a date automatically updated with every deployment.
See also: Index bloat, Crawl budget, Orphan page.
