Civilizations are playable factions, each of which represents a historical nation, empire or cultural group. Each civilization in Civilization VI has a unique ability and two unique components: a unit and a piece of infrastructure, which may be a building, a district, or a tile improvement.
Civilization VI makes a distinction between a civilization and its leader, not unlike Civilization IV - each civilization can now have multiple leaders, and each leader comes with his or her own unique ability and pursues a specific agenda throughout the game (when controlled by the AI). Some leader abilities also provide an additional unique unit. So, the civilization's unique unit, infrastructure and ability remain unchanged, while the leader ability and agenda are affected by the leader chosen. Thus the same civilization may behave quite differently throughout the game, depending on which leader leads it. A wise player will learn not only all different civilizations' special abilities and units, but also their leaders' special traits and agendas, which will allow him or her to anticipate the AI behavior during the game.
Starting a Trade Route immediately creates a Trading Post in the destination city. Having a Trading Post in any city of a civilization grants 1 extra level of Diplomatic Visibility with that civilization. Mongolian Units receive a +6 Combat Strength bonus for each level of Diplomatic Visibility over their opponents, instead of the usual +3.
Conquering a city will upgrade the conquering unit into a Corps or Army, if the proper civics are unlocked. +3 Loyalty per turn to cities with a garrisoned unit, increasing to +5 Loyalty per turn if the garrisoned unit is a Corps or Army.