Google Ignores Irrelevant Sitemap Content
Google will ignore any information in Sitemaps which it doesn’t recognise, so you can include additional information for other purposes.
No Good Solution for Reactivating Pages
If you have pages which expire but are reactivated after a period of time, there isn’t really a good solution, but you can use a Sitemap to tell Google about URLs which are now active, and use the unavailable-after meta tag.
Mobile Sites Don’t Need Sitemaps
Separate mobile sites should be canonicalising to the desktop page, so you don’t need to submit them to Google via a Sitemap, but it’s still worth adding to Search Console.
HTML sitemaps help indexing and crawling
If you have a complicated website, providing a mapping of your category pages can help Google to find pages and understand the structure of a website.
Cross Domain Sitemaps Will Be Crawled If Present in Robots.txt
Google will use Sitemaps hosted on an external domain if they are referenced in the robots.txt.
RSS + PubSubHubbub is better than XML sitemaps
John recommends using RSS with PubSubHubbub as the fastest way to get new content indexed.
Submit Canonical URLs in Sitemaps and Other References
Use consistent canonical URLs in Sitemaps and other internal references.
Use Sitemaps for Redirect Discovery
If you want Google to see your redirected URLs, such as after a URL change, it’s OK to submit the old URLs in a Sitemap to help Google recrawl them more quickly
Break XML Sitemaps into Small Chunks
Breaking up XML Sitemaps into smaller groups can give you more feedback on indexing issues, which are reported separately for each Sitemap in Webmaster Tools.