What is an appendum?

Answer 1

An addendum is extra information that an author or writer discovered after writing a book, report or document, such as a new study on the topic.

The American Heritage Dictionary defines an addendum as something added or to be added, especially a supplement to a book.

Similarly, an erratum or errata is a mistake in a book that is discovered after it was printed and is then corrected in a list added to the book or the publisher/author's website.