Addendum and Extra

Addendum noun - A part added at the end of a book or periodical.

Extra is a synonym for addendum in addition topic. In some cases you can use "Extra" instead a noun "Addendum", when it comes to topics like added.

Extra noun - An interchangeable part or piece of equipment that is kept on hand for replacement of an original.

Addendum is a synonym for extra in addition topic. You can use "Addendum" instead a noun "Extra", if it concerns topics such as accessory.