XML sitemaps are one of the best ways to tell Google and other search engines about your website pages that you want them to index. Google Search Console (Webmaster Tool) asks you to submit the sitemap every time you submit your website. So, its important for you to ensure that you make your XML sitemap error free and compatible to Webmaster tool guidelines.
XML sitemap contains the below important metrics:
1. loc: This tells the absolute path of your website pages. This is the most crucial element for an XML sitemap.
2. lastmod: This tells when your page was last modified/updated. This is optional field and Google doesn’t give this much preference. According to Google:
The lastmod tag is optional in sitmaps and in most of the cases it’s ignored by search engines, because webmasters are doing a horrible job keeping it accurate.
3. priority: This tells how you give priority to your website pages. There was some lack of clarity for this parameter if passing this information impacts rankings in any way or if Google actually uses this information anywhere. Well, it is now made clear by Google’s Gary Illyes on Twitter that Google ignore this tag as well.
we ignore those. It's essentially a bag of noise
— Gary 鯨理／경리 Illyes (@methode) March 28, 2017