Rider

A rider is a document attached to a contract to add or modify specific terms, often standardized ones layered onto a base agreement. The term is common in insurance (a rider adds coverage to a policy) and in entertainment and real estate, but the idea applies anywhere: a base contract plus a rider that tailors it to a particular situation.

Functionally a rider overlaps with an addendum. Whatever it is called, the key is that it is signed by the parties and clearly states how it interacts with the main agreement.