He explains with examples: Let's say that you own www.example.com and mysite.google.com and you have Sitemaps for both hosts, e.g. sitemap-example.xml and sitemap-mysite.xml. Until today, you would have to store each Sitemap on its respective host. If you tried to place sitemap-mysite.xml on www.example.com, you would get an error because, for security reasons, a Sitemap on www.example.com can only contains URLs from www.example.com. So how do we solve this? Well, if you can "prove" that you own or control both of these hosts, then either one can host a Sitemap containing URLs for the other. Just follow the normal verification process in Google Webmaster Tools and any verified site in your account will be able to host Sitemaps for any other verified site in the same account.
Here is an example showing both sites verified:
And now, from a single host, you can submit Sitemaps for both sites without any errors. sitemap-example.xml contains URLs from www.example.com and sitemap-mysite.xml contains URLs from mysite.google.com but both now reside on www.example.com:
For those of you wondering how this affects the other search engines that support the Sitemap Protocol, rest assured that we're talking to them about how to make cross-submissions work seamlessly across all of them. Until then, this specific solution will work only for users of Google Webmaster Tools.
Sitemap, Sitmaps, Google, Tips and Tricks, Knowledgebase, Article