What does "pre-existing content" mean?

During the copyright registration application, Cosynd will ask you if there is any pre-existing content in your copyright that was not created by you or the other author(s).

Pre-existing content refers to material that is not created or owned by the authors you have listed, such as a sample of another song, an excerpt of another book, stock photos and footage etc.  If the copyrights you want to register contain any pre-existing content, you should check "Yes" here. 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.