ERN 3 explained
The Electronic Release Notification Message Suite in version 3 contains three messages
NewReleaseMessageCatalogueListMessagePurgeReleaseMessage
The specification in version 3.8.2 (the most commonly implemented version) can be found here.
The most recent version of the ERN to be published is ERN 4.3 and DDEX strongly advises that you implement this version of ERN, rather than a version of ERN 3.