How Trade works

[Servius]

On Supply...
- Trade goods (Furs, Tobacco, Cotton, Sugar, Spices, Ivory, Tea, and Coffee) are generated in your provices or by trade ports on the coasts of South America, Africa, and Indonesia.
- The number of units generated by plantations is determined first by the level of the plantation (which will give you the base units produced), which is then modified by the site quality. Meagre = 100% of listed production, Low = 125%, Average = 150%, High = 175%, and Abundant = 200%. Thus, it is always more cost effective to upgrade plantations on Abundant or High producing towns than for Low or Meagre ones.
- There is also text on the trade port page that says upgrading ports does something to improve the "trade value" but I can't nail that down to increasing the world price, increasing the number of units, or increasing the "Other Goods" category. My bet is that trade port levels determine the max amount of "Other Goods" you trade with your partners. Can't confirm yet though.
- If one of your export trade ports gets blockaded or raided, it can't export any goods. If you're an island with only one trade port, no goods get out. If you're on a contenent with other trade ports in other regions next door, or if there are more than 1 trade port on your island, the goods will just get shipped out through another one of your trade ports. This is true, despite the impression the Trade window gives you that no goods from the entire theatre are being supplied to your home region.
- The trade routes are those colored dashed lines that run between ports. When you park a ship on one that carries trade goods for a nation you're at war with, you'll steal some of the goods or income the other nation would have recieved. Income from raiding other nations' trade routes shows in up as "Other" in the revenue list (where you see your tax income and your trade income), along with Protectorate income, etc. You can raid intra-national routes (like one from Jamaica to England), which leads first to a reduction in the supply of trade goods to England, and thus results in less trade revenue for England. You can also raid inter-national routes (like from England to France), which does not affect the supply of trade goods but does affect the income received from trade partners.
- Every trade good generated by one of your regions or those coastal trade posts gets shiped to your capital. As GB, everything goes to England first.
- Before the most recent patch, 100% of the trade goods generated by a region could be sent to the nation's home region for later export with even the level 1 trade port in the supplying region. After the patch, the supply of trade goods to the home region can be reduced if your trade port is too low level. I haven't figured out the number yet, but each trade route can carry x goods. So, a fully-upgraded Meagre plantation can have all of it's exports handled by a level 2 or 3 trade port, but a fully-upgraded Abundant plantation will require a level 4 trade port (and maybe part of a second trade port's capacity) to export all of the units it produces. Open up your region capital's detail screen, look at the Region Wealth section, and run your cursor over the trade good icons there. That will tell you how many of the trade goods produce in that region are actually being supplied to your home region for export to trade partners.

On Exports...
- The goods that reach your capital are then sold to your trade partners at the global prices shown at the top of the Trade window. I don't know what determines how much goes to each trade partner, but my guess is the main (if not sole) variable is the population of the trade partner. The bigger their population, the greater part of the pie they get.
- This is why the number of trade partnerships you can have is partly determined by the number of trade ports in your home theatre. READ AGAIN: HOME THEATRE. So, using GB again, upgrading trade ports in Ireland will grant you more trade partner slots. So, it's not just trade ports in England or even Scotland.
- As metioned above, inter-national trade routes can also be raided. When these are raided, the trade income you get from your trade partners goes down, even if your supply of trade goods stays constant.