DDEX provides naming conventions in two main areas: The XML composite and tag names, and allowed-value sets (aka code lists)the Allowed Value Sets.
The XML is defined in the Schema files that can be downloaded in a zip from https://kb.ddex.net/display/MLC13. The zip file contains two files: the main XSD defining the messages and composites of the MLC Message Site Standard and an XSD for the allowed value sets.The message XSD can also be accessed from available under a link from here.
The DDEX Allowed Value Sets are specified at http://service.ddex.net/xml/mlc/13/music-licensing-companies.xsd and the XSD for the allowed-value sets can be accessed from http://ddex.net/xml/avs/avs_20161006a.xsd.
avs/avs with the AVS that are specific to the MLC Message Suite appearing together within a section of the document at the above link.