保险单和保险证书的作用是相似的，它们之间的唯一差别就是后者比前者更简单一些。

已装船提单表明货物已实际装上开往目的港的承运船只。

in a broad sense

per-unit

private carrier

logistics

cost economies

契约承运人

common carriers

Ocean freight is the most widely used form of transportation in international trade. As the most efficient form in terms of energy, ocean freight has its attractions in delivering large quantities of goods over long distances. Compared with ocean freight, air freight enjoys the most obvious advantage in the speed of delivery. Another benefit of air transport-greater security helps to reduce insurance premium since the risk of pilferage and damage is much smaller.With the development of international trade, air traffic has increased rapidly.