An author copyright addendum is a legal tool that changes a publishing agreement and allows the author to keep key rights in their articles.  The addendum is presented to a publisher at the time a publishing contract is being negotiated and signed to spark a dialogue about rights ownership issues as between the author and the publisher.  Examples of an author copyright addendum and more information on author copyright addendums can be found:

• SPARC* - https://sparcopen.org/our-work/author-rights/#background
• Scholar's Copyright Addendum Engine - https://labs.creativecommons.org/scholars/