A file listing multiple sitemaps, allowing a website to organize a larger URL inventory.
An XML sitemap index is a file that lists the locations of other sitemap files. Each child sitemap then lists content URLs. This extra layer lets a site organize its URL inventory into manageable groups without making the top-level file a direct list of every page.
A publishing system might separate posts, products and other pages, or split a large inventory across several files. The index points crawlers to those files. Its purpose is organization and discovery; using an index does not create more pages, improve their content or guarantee that search engines will include them.
The distinction matters during verification. Successfully opening the index only proves that the first layer is available. A child sitemap might be missing, return an error or omit recent pages. Inspect the relevant child file and a sample of its content URLs before deciding that the sitemap setup represents the site correctly.
Takeaway: When checking a site with multiple sitemaps, follow the chain from the index to the relevant child file and then to the actual pages. A healthy top-level response alone is not enough.
Example
A publisher’s sitemap index lists one file for articles and one for service pages. The service file loads correctly, while the article file still points to a previous address after a migration. A check limited to the index would miss the problem. Following both child links makes the remaining migration issue visible.
What to check
Google documents size and entry limits for both sitemaps and index files. Let the site’s supported generator handle splitting where possible, then check its output. Keep the inventory meaningful; splitting a small sitemap into more files is not a ranking tactic.
For more detail, see Manage your sitemaps with sitemap index files.