The Sitemap Index
Breaking up the Google Sitemap, or the News Sitemap, into multiple documents is a task that SiteTree carries out automatically as soon as you include in the sitemap more than one type of content (Posts, Pages, Categories, etc.).
The break up of an XML Sitemap is always accompanied by the creation of a Sitemap Index, a document from where the whole set of sitemaps can be located.
The Sitemap Index
A Sitemap Index is a document collecting all the URLs of the sitemaps in which a single Google Sitemap, or News Sitemap, has been split, and can link to up to 50k sitemaps. Such a document makes submitting to Google changes made to one or more sitemaps a straightforward operation, since just one ping is needed.
The Sitemap Index produced by SiteTree is a dynamic file accessible at the same location as that of the single sitemap it replaces, so you don't have to submit a new URL through the Google Search Console.
Content Types as Key to Organisation
Keeping sitemaps organised isn't just a matter of personal taste, but can really help you in noticing if something is going the wrong way. That's why SiteTree creates a separate sitemap for each type of content, by further splitting the sitemaps when the amount of permalinks for that content type is really generous, so as to actually create groups of sitemaps.
Controlling Sitemap's Capacity
Although SiteTree is completely autonomous in creating and managing multiple sitemaps, you have however control on the limit about the number of permalinks each Google Sitemap, or News Sitemap, must contain: you can unhide this setting by clicking on the 'Configure' buttons in the SiteTree Dashboard.
At the time of writing, SiteTree can be set up to generate News Sitemaps listing up to 1k URLs each and Google Sitemaps with a maximum capacity of 10k permalinks each.