Which direction matters after a redesign?

Pages that exist and are crawlable but are missing from the sitemap. A redesign can drop a template out of sitemap.xml while the pages still answer 200, so search engines only find them through links, later and at lower priority.

What does "missing from the sitemap" mean?

The URL is not listed in sitemap.xml at all. The page may be perfectly reachable; the point is that the file search engines read does not mention it, so discovery depends on links alone.

Why would a sitemap URL not return 200?

The page was deleted, moved, or renamed and the sitemap is stale. A sitemap that lists pages that 404 (or redirect) sends crawlers to dead ends and wastes the budget you want spent on real pages.

How many pages does it check?

A bounded sample: the homepage, the links reachable from it and from a set of sampled pages, and a spread selection of sitemap URLs. It is enough to catch a template or a whole section that fell out of the sitemap, not an exhaustive export of every URL.

Which pages is your sitemap missing?

Paste your address. This reads your sitemap and crawls a bounded sample of the site, then compares both directions: pages that exist but are absent from the sitemap, and sitemap URLs that no longer answer 200.

Reads /sitemap.xml and a bounded sample of the site. Nothing is stored.

How this works

Your sitemap is read the way a crawler reads it, then a bounded sample of the site is crawled through its own links. A page the crawl reached but the sitemap does not list is reported as missing from the sitemap; a sitemap URL that does not return 200 is reported the other way. This is the reverse of the orphan-pages check, which starts from the sitemap and looks for pages nothing links to.

Questions

Which direction matters after a redesign?: Pages that exist and are crawlable but are missing from the sitemap. A redesign can drop a template out of sitemap.xml while the pages still answer 200, so search engines only find them through links, later and at lower priority.
What does "missing from the sitemap" mean?: The URL is not listed in sitemap.xml at all. The page may be perfectly reachable; the point is that the file search engines read does not mention it, so discovery depends on links alone.
Why would a sitemap URL not return 200?: The page was deleted, moved, or renamed and the sitemap is stale. A sitemap that lists pages that 404 (or redirect) sends crawlers to dead ends and wastes the budget you want spent on real pages.
How many pages does it check?: A bounded sample: the homepage, the links reachable from it and from a set of sampled pages, and a spread selection of sitemap URLs. It is enough to catch a template or a whole section that fell out of the sitemap, not an exhaustive export of every URL.