Export/Import transactions require a contract of sale.
Reaching a sales agreement involves a flow of form documents between an exporter and an importer.
The sales agreement (or contract) may be a single document or may be created by an exchange of form documents between the parties to the trade.
The most common form documents are the seller's "pro forma invoice" and the buyer's "purchase order", often accompanied by legal conditions known as general conditions of sale (or purchase).