Cargo is used to make Automated Transport Routes between your cities. Later in the Game you can create those Routes between players with a special tech that unlocks Import and Export to and from other Players cities. You need a Trade treaty with another player to make automated Trade routes. This building only requires Workforce(Adult Pigs) to produce its Resource.
Cargo is a Resource produced by 2 types of buildings.
Be aware that Caravanserai can only be built on land and Docks only at the shore. If you are able to build both, you should stick to Docks since they provide more cargo capacity then Land Docks.
Each resource has its own cargo cost per unit
Here is a chart showing the cargo cost per unit of the resource
|Resource||Cargo cost per unit|
Cargo costs are multiplied by the average distance between source city, destination city and cargo city.
In the below example, a trade route between the RED cities has a distance of 6. If cargo is provided by either city the cost factor would be 6+6/2=6. If the cargo was provided by the PURPLE city, which has a distance of 3 between both, the cost factor would be 3+3/2=3.