The definitive version of the schema is msdesc.rng in the parent folder. That contains additional schematron rules to encourage best practice in cataloguing.
But the XSD and DTD versions in this folder are provided for users with systems that do not support RELAX NG. They have been generated from the same msdesc.odd file.
The direct URLs for validating (e.g. to paste into an xml-model declaration or to point Oxygen XML editor at) are: