Diplomacy in the game may not be as important as in other games of this genre, but it remains a pretty important aspect of the gameplay. With this, you will be able to trade with other factions, maintain friendly relations (or sign military contracts) or even assimilate other factions to your kingdom. This is just another symptom of the larger problem called „Total War Diplomacy,“ something that has shown very little love and attention since CA was first born. They even took poor little diplomatic boats and cars on the campaign map. They waved their finger against diplomacy and also took trade routes. Tilea and Estalia are also very difficult to obtain as business partners for reasons I can`t understand. You can only negotiate with other groups if you have produced at least one commercial resource (iron, stone, wood, etc.) in one of your colonies and the fraction with which you want to trade does not yet have it. The value of each resource is different (you can check it in the bottom left corner of the diplomacy screen). In addition, the amount of gold you earn through trade depends on the amount of resource you can produce (the higher the level of the building it can manufacture, the more gold you will earn with that). Each resource you export increases your group`s income, while each imported one increases your trading partner`s income. Unfortunately, you can`t decide what resources you want to send them and what resources you want to receive.
In the late game, it causes situations where you can win 100 gold from the trade, while the other faction 600 or more. However, it is always worth doing, because you will not really lose something and trade increases relations with a political group. It also depends on the characteristics of the political groups. A group like Kislev, which has passive quality, is by no means to act. Note: The Tomb Kings are the only faction that actually uses resources for their mortuary cult mechanics (Edit: a similar mechanic called forgery has been added to the dwarves in Warhammer 2 only since this response was originally published). For all other groups, resources only increase the value of trade agreements. So you`re generally right to think that these resources are not actually used by the trading partner, but not in this particular example. On the left (on your side), the interface shows you how much you earn with the trade and resources you export to your trading partner. The right side shows how much money they earn and what resources you import from them.
It works in a rather strange way. By signing a trade agreement, you send all your resources to the group you are acting with (except those the group already has) and vice versa. A simple example will help you understand better: I have never seen evidence that more commercial resources would increase the likelihood that someone would accept a trade agreement. The diplomatic system is very veiled here. I have seen that groups with 200-plus relationships are opposed to profitable trade for them and that groups with 5-strong relationships accept the same trade. Trade can take place between two political groups in campaign mode.