Trade Interest is a temporal diplomatic variable. Its purpose is to emulate the mood of the AI wizards : If they have been bombarded with too many trade offers, they get tired of it and will be less likely to accept any such offer. Trade Interest is not symmetric. In relations between the human and AI players, only one side of the variable pair is being actively used, but in AI to AI relations, either side can be used depending on which AI player is offering the trade to the other.