The XML Schema files for all DDEX standards are available in two forms:
Implementers can download a zip archive with all relevant XML Schema definitions from the same page where the relevant standard itself is available for download.
These files are specifically designed to be downloaded and used when implementing the relevant DDEX standards.
Implementers can also access all XML Schema files from the appropriate subfolder from http://ddex.net/xml (e.g. http://ddex.net/xml/ern/351/ for Version 3.5.1 of the Release Notification Message Suite Standard (a.k.a. ERN).
It is these locations that users of the standard are expected to use in the top level tag of a DDEX message exchanged commercially: