Copyrights to a song or any piece of sound recording are typically broken down into several ownership rights. They represent different roles in the making of that song, even if no one else was involved in the creation of your soundtrack. It just means you assume all these roles including owning the different copyrights every soundtrack comes with.
What most people don't know is that these different roles all get paid separate royalties from each other. Also meaning that when the use of your song is played once, it generates several different royalty streams. For example 1 stream play generates a minimum of 4 separate royalties.
Publishing rights are the rights usually transferred from the original copyright holder over to a publicist, whose task and responsibility is to exploit your music, which allows publicist to reproduce, publish and license your music to generate royalties and revenue for the original rights holder.
Like a book publisher after you’ve written your book, publishing copies of your original written works to generate book sales and revenue, for you being the author.
However it is common practice in the music industry, for recording artists to have all their music's publishing rights assigned to their publicist, relinquishing these rights in a publishing deal. Granting their publicist nearly 50% of royalties to every song they have them publishing.