What does this report contain?
URLs which were found within the mobile/AMP rel alt tag of another page, but which return a non-200 HTTP status code
How can this report be used?
URLs which are rel alted from another page should return a 200 HTTP status code. Error pages (4xx/5xx) will mean that the alternate page is unable to be indexed. A 3xx status code may still mean the destination URL will be indexed (assuming it has a correct reciprocal link), however will unnecessarily consume crawl budget.