Interlineation is a legal term that signifies a sentence that is newly inserted between previous sentences in a contract

Contract

A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...

both parties had agreed upon previously. Interlineations help clarify specific points of a legal contract, and are useful if some of the terms of a contract become unclear later. They are typically initialed and dated, so as to make a record of changes and agreements to a particular contract.