During the copyright registration application, Cosynd will ask you if there is any material in your work that was not created by you or the other author(s).
"Material" refers to the individual pieces of content that make up the entire work. For example, the parts of a song such as the sound recording, lyrics, text, and arrangement are all considered to be different "materials."
If the work you are trying to register includes material that is not created or owned by the authors you have listed, Cosynd will ask you to identify these parts later on in the process so that the U.S. Copyright Office will not include them in your registration.