An appendix supplements the body of a document, providing detailed information that not everyone will want to read. Appendices are often statistical, historical or technical. An addendum is extra information that the writer discovered after writing the report, such as a new study on the topic. It’s a bit like a PS.

An addendum is an add-on that can be created at any point, even years after the original contract was drafted, as long as all parties agree to it. An appendix is created at the same time as the contract and provides extra information or details that are useful (though not critical) to know.

An addendum is typically used to add or update information that was not included in the original document, while an appendix is used to provide supporting or supplementary material that is relevant to the main content.

Addendum and Appendix are both supplementary materials added to the end of a document, but they serve different purposes. Addendum provides additional information or clarifications, while appendix supports the document's content with detailed information.

The main difference between addendum and appendix is, an addendum contains new information that the writer discovered after writing the text while an appendix contains extra information that is useful to the reader.