This is about the rights of an actual song, not a recording of a song. So it concerns not the performer(s) of a song but the composer(s)/lyricist(s) of the song. There are several ways publishing revenue is generated. The first is when a song is performed (played on radio, performed live, shown on TV etc.). This is what we call performance royalties. The second one is a fee for every reproduction of a song (download or on a physical product like a CD). This is what we call the mechanical royalties. A third important one is licensing royalties. These are generated when a song is licensed to be used in, for instance, a movie, TV commercial or game.