In the ReleaseList of ERN-3, it is in many[1] cases not only correct but mandatory to include multiple Releases:

The Main Release is identified by the ReleaseType value of, for instance,  "Album" (for traditional albums) or "Single" (typically used for Releases with one track released in a distinct package from an album, or occasionally a few tracks).

It is, however, incorrect to use the value "Single" for the track-level Releases. These must instead be identified by the ReleaseType of "TrackRelease" or  "VideoTrackRelease".

ERN-4 does not support different types for Track Releases.


[1]  There are a number of profiles that do not allow any Track Releases (Ringtones, MidiRingtones, Long-form Music Videos, TV Series Releases, Film Bundle Releases, Wall Paper Releases, Simple Video Single and Single Resource Release). In those cases, an ERN message would typically only contain one Release.