Climate represents the conditions of a planet. The more favorable a climate, the better the population growth and happiness increase.
Most races can grow on temperatures that range between 15 and 84. The farther away from 50, the slower the growth and the harder it is to raise happiness.
The Crystal Confederation, on the other hand, grows best on 100, and grows slower on cooler temperatures. Nonetheless, they still have a cap advantage on any arctic climates above 0 except for The Rebel Confederation.
Outside of these favorable conditions, the colonists cannot grow on these planets, and planets have a low base population cap. What is not immediately obvious is that clans on the surface of a planet in excess of the maximum can eat supplies (a small amount) to live, provided that there are enough supplies on the planet.
For every extra clan over the maximum population, four supply units must also be on the surface of the planet to ensure their survival. This can occasionally be difficult to arrange. An Avian planet requiring 1200 clans over maximum population will need a staggering 4800 supplies! Yet this will pay itself back quite quickly with patience, because it raises the maximum amount taxable from the natives per turn.
The Rebel Confederation has a significant cap advantage over any other race regarding population caps on any planet below 20 degrees. They can support 90,000 clans with no ill side effects.