Answer:
A rider.
Step-by-step explanation:
A rider designates a clause or group of clauses added to an initial contract, being an annex to it. These added clauses are attached to the initial contract, integrating to it and losing the character of an independent document. Therefore, it would then be a piece of writing that is to be treated as part of the main contract. Usually, the purpose of the riders is to include particular clauses for the insured, that is, clauses that are specific to them outside of the company's generic contract.