The Sitemap Index
Breaking up the two XML Sitemaps produced by SiteTree into multiple documents is one of the raisons d'être of Demux Leaf, that carries out the task automatically as soon as a target number of permalinks is reached, and in the context of the News Sitemap, also when the News Sitemap contains more than one Post Type.
An advantage of using multiple sitemaps is to lighten up server work: lowering the time each sitemap can be served in can make your sitemaps more crawler-friendly.
Usually, the break up of an XML Sitemap is accompanied by the creation of a "place" from where the whole set of sitemaps can be located, a Sitemap Index. And that's where Demux Leaf comes in again.
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 Demux Leaf is a dynamic file accessible at the same location as that of the single sitemap generated by SiteTree, 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 Demux Leaf 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 Demux Leaf works in complete autonomy, 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.
With Demux Leaf installed, 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.