An order is a stated intention, either spoken or written, to engage in a commercial transaction for specific products or services. It expresses the intention to buy and can be called a purchase order. 

A delivery contract should be used to record the negotiation of an agreement to buy and sell, record the agreement itself, and record compliance with the agreement and closure of the contract. An agreement to buy and sell is a form of contract.

Five basic requirements for a contract should exist between two parties: agreement, voluntary, consideration, capacity, and legality. 

Once an agreement or contract is in place, businesses record these as confirmed purchase orders and confirmed sales orders.