Colonists are the representation of the player population. They can grow, be taxed, colonize planets with the assistant of starships, and collect taxes from natives. 100 colonists equals 1 clan, which in turn requires 1kT of a starship's cargo hold.
Colonists Growth and Maximums
Outside of the favorable, the planet is considered a hostile world. No growth can take place. Default Climate Death Rate (or simply CDR) is 10. If the climate is less than 15, then the planet is considered an arctic world, and the formula for maxA applies. Otherwise, if the climate is at least 85, then the planet is considered a desert world, and maxD applies.
There are a couple of notable exceptions towards these maximums. Fascist, Robotic, Rebel, and Colonial owned planets are always able to support 60 colonists on desert worlds, so if maxD is calculated to be less than 60, then the 60 takes precedence.
Likewise, Rebel planets can support 90,000 clans if the climate is less than 20, although they still cannot grow if the climate is below 15.
In any case, if the population is more than 6.6 million (or 66,000 clans), then growth is halved. Additionally, if the colonists are growing, then it will not exceed the maximum population allowed by the climate