Center of Trade
From EU3 Wiki
A Center of Trade (CoT) is a major trading hub for surrounding provinces. The trade goods from these provinces go through the CoT, and merchants there profit from it. Use the Trade Mapmode (lower right corner of screen) to see all provinces that contribute to a particular CoT, or hover your mouse over the "Total value of trade" in the CoT window to see the production income from these provinces.
The owner of a CoT's province gets 10 ducats' tax income from it.
A new CoT will appear on the map each time a certain threshold of new colonies/provinces is passed. If you are playing a game where you attempt to spam the world with new, unfinished colonies (to block others from taking the land), note that you should try to build a very light sprinkling of them to a full city. If you don't, new CoTs will appear in other nations' nearby provinces that do have cities. The game tries to give new CoTs to the person with the most provinces in the area (including colonies), but if none of them have cities, you're out of luck.
