This document outlines the XSD document is basically a schema or rule-set document written in the XML mark-up language. This XSD describes how the XML data should be structured when it is extracted from the form. Once a form object has been bound to the schema elements, the data captured in the form becomes stored in an XML document in the format as described by the XSD.
In Summary -
XML Schema definition also known as the XSD.
XSD used for data binding in XDP as well as Adoptive form.