Most of the musical works and sound recordings for which metadata is communicated between business partners, using the ERN standard, is owned by one or, typically, multiple rights holder(s). This applies to both the rights in the recording and the rights in the underlying musical work. In some cases, however, such rights might have expired in one or more territories.
The ERN message was not specified to convey any information about the current rights holders of the musical works used embodied in Resources (and, thus, used in Releases). As a consequence, there is no means to determine, from an ERN message alone, whether a musical work embodied in a Resource is in the public domain or, if it is, in which territories it is in the public domain.
"Public domain" is a term used to denote that a musical work (or indeed a sound recording) is no longer protected by copyright in a specific territory, usually as a result of the effluxion of time.