Hi, Eileen! My impression is that the text is talking about registering musical works with an organization that oversees copyright issues. In this case, it appears to be a specific arrangement of a work that has been up until that moment in the public domain. The implication would be that the new musical arrangement makes it an original work based on the previous one, thus necessitating registering it with the copyright office or the like. This would be something like registering a translation. The original book might be in the public domain, but your translation of it would need to be copyrighted.