There are two separate entities in a musical copyright. The first one covers the composition, or the idea, of the song. Meaning the melodies, chord progressions and lyrics (if any). The second one covers the actual recording of this song.
To illustrate it, the first copyright would be like the FLP file (if you use FL Studio) This file contains the written melodies/chord progressions, the blue print of the song. The second copyright would be like the exported WAV file since it is the recording of all the content together in it’s final form.
The first copyright is called the Performing Arts copyright (or PA Copyright) and the second is called a Sound Recording (or SR Copyright). Every song can have only one Performing Arts copyright, but it can have multiple Sound Recording copyrights. Let’s look at why a given song can have multiple Sound Recording copyrights.
Let’s say that you solely created an entire song. When you register your song you own both the PA and SR copyright. If someone does a cover of your song they can register their own Sound Recording copyright for it. They can’t register a Performing Arts copyright because there can only be one, and the Sound Recording copyright that you own is also called the “Master Recording” since it is the original.