This is an Orbeon Forms PE feature.
The toolbox's "XML Schema" button under the "Advanced" tab allows you to attach an XML Schema to the form. This serves two purposes:
The schema is used to validate the form data.
Simple types present in the schema are made available in the Validation Details dialog.
NOTE: Form Builder does not handle imported/included sub-schemas. If you attempt to attach such a schema, a warning message is displayed.