If business is now a global affair, then the export-import business is important, lucrative and necessary. An export-import agent is not a broker --- the agents are lower on the business totem pole.
Most properties are measured using a number and a unit of measurement. The unit of measurement is a standard amount. The number compares the property to that standard amount. This means a tree that is ten times as tall as a meter stick is 10 meters tall.
A unit is a measure of something, i.e. centimeters are a unit of length, cubic inches are a unit of volume and pounds are a unit of weight. Units are how you specify what a number represents physically.