Population unit is an unit that represents how many people live in a country. Different kinds of tiles give different kinds of population units. Normal tiles hold 1.0 PU, Mountain tiles hold 0.75 PU, and desert and tundra tiles hold 0.5 PU. Uninhabitable tiles and sea tiles hold none, but freshwater lakes multiply the PU of surrounding tiles by 2 (if they're in the same country) or by 1.2 (if they're a different country).Coasts give 0.1 PU.