built-in implementations of the persistence API, that XML document is also saved in your database of choice.
The structure of those XML documents is driven by the names you give to sections, repeats, and controls when you create your form in Form Builder, so you know what the XML will look like. But what if downstream, software is consuming those XML documents? A better way to communicate what the XML "looks like" to software is by using an XML schema. And this is why Orbeon Forms 4.0 PE introduced a new service which can automatically produce an XML schema describing the structure of the XML data collected by each form you built with Form Builder.
The service that produces the schema is accessible at /fr/[app]/[form]/schema for versions 4.0 to 4.3 and at /fr/service/[app]/[form]/schema for versions 4.4 and newer. Like other services, since 4.0, it is protected by default, and thus isn't just accessible by end users trying to load that URL.