An amendment is typically used to change something that's part of an original contract. Think of amendments as modifications to the earliest agreement (for example, altering an agreed-upon deadline). An addendum is used to clarify and add things that were not initially part of the original contract or agreement.

What is also known as an addendum?

An addendum or appendix, in general, is an addition required to be made to a document by its author subsequent to its printing or publication. ...

What is the legal term for addendum?

addendum. n. an addition to a completed written document. Most commonly this is a proposed change or explanation (such as a list of goods to be included) in a contract, or some point that has been the subject of negotiation after the contract was originally proposed by one party.