A contract addendum is a document that is added to an existing contract to modify the terms and conditions of the agreement. An addendum is typically used when there are changes or additions that need to be made to an existing contract, but the original agreement has already been completed or put into effect. Addendums are usually drawn up as part of the negotiation process between two or more parties who have previously agreed to most of the key terms in the contract.

On the other hand, a contract amendment is a document that is used to make changes to existing contract terms, conditions, or scope. Amendment documents are typically used when there has been a change in circumstances since the original agreement was signed and the parties need to update the contract accordingly. The amendment must be mutually acceptable by both parties and can include changes such as adjustments to payment amounts, revisions to service levels, or the introduction of new clauses – any slight altering of the original terms.