Trade is the exchange of resources between settlements. In Might & Fealty, the player characters as lords do not bother with individual shipments, but with constant supply, called trade deals.
The daily buying and selling is assumed to happen all the time, being handled by travelling merchants. This background trade is the reason that construction can progress at a slower speed, even if a critical resource is unavailable - it is assumed to be unavailable as a reliable flow, but needs instead to be bought on demand from whatever supply merchants can provide.
The basic trade system is very simple: Trade is simply settlement A sending a quantity of resource X to settlement B.
And that is all.
Out of this simple concept emerges all the complexities of the trade system. For example, a one-sided trade can be considered a tribute payment, while a reciprocal trade, where A sends something to B and B sends something else in return is a mutual trade deal.
There are no restrictions or limitations on trade except the natural limits (you cannot trade what you do not have).
This opens the door for any deal imaginable. The exchange rate of resources is a free market and you can get whatever you can negotiate. There is no money system in Might & Fealty, and intentionally so. You trade food for metal or wood for goods, or goods+wood for food or maybe you just trade food for the promise of not burning them down.
Shipping resources around is not free, but incurs a trade cost, which depends mostly on the distance between the two settlements. It increases in a non-linear matter, which is a fancy way of saying that if A and B are very far away, and C somewhere in the middle, then a trade A->C->B would incur a much lower summed trade cost than the direct A->B trade. This makes trade centers possible, allowing settlements to act as middle-men for long-distance trades, at a profit to everyone involved. Of course, such a deal requires mutual trust.
Merchants in your entourage will reduce the trade cost, and one merchant will stay behind to oversee the details.
Creating new trade deals can be done when you are physically present or when you are answering an existing trade deal. In other words: