The content store and delivery layer behind the documentation and marketing surfaces.
Content is held separately from the applications that render it, so a page's text is edited without a deployment. Site reads from it for product overviews and quickstarts; the marketing surfaces read from it for everything else.
Keeping content out of the build is what allowed documentation edits to move at a different speed from code, which is the only speed that works for either.
One endpoint is recorded against it, which is the honest measure of how thinly this slot was documented rather than of how much it did: the marketing site and the documentation surface both read from the same content service, and neither of them wrote its contract down.