There was an interesting question regarding the location directory, the XML sitemap is placed in, asking that does it matter to Google and if yes how?
XML sitemaps are a very powerful tool. Google does not index your pages just because you have said it. Google Indexes a page because it has found and crawled through it, and has found the matter in good quality and worth indexing.
When you submit an XML sitemap to Google, it is a clue and indication to consider those pages as they have good and important context to be reached to our users. Many people make the mistake of not informing Google about the pages. In that case, all your content goes to waste. For example, your site has 1000 pages out of which only 5-6 pages have relevant and important. when Google links a user to your website, what will be the user experience …It will be a very poor experience as he may land up on any page that does not have anything important to read. Hence, why would Google ever want to send his users to such a site?
When you actually want to tell Google about your site, you need to be specific that out of 1000 pages, only 400 pages have good content and you want them to be indexed, rest are just utility pages. Google will go through those 400 pages and with the help of their metrics; they may decide that only 200 pages are A grade, rest 100 may be B or B+ and so on.
On a discussion on twitter, John says that if the MML sitemaps were submitted via an anonymous source, they will be valid only there and to lower levels. However, if you submit it through search console account. It is valid for any verified site. If you have an XML sitemap and it is located in its subdirectory, then it is valid for only those URL’s that are present in that subdirectory. In other cases, if you submit a file through search console then you will be able to track its submission, you can track if it is processed. You will also see a lot of URL’s that are indexed.
I would conclude by sayings that use your XML sitemaps tools to discover and eliminate indexing problems. Ask Google to index only those pages that you are sure and you know Google will want to Index.