Our rarity elements are based on real world rarity. For example: Glaciers are rare landscapes because they will increase in rarity in the future. With global warming, we are unsure if most of our glaciers still exist in a matter of years. When we look at skies, we see that eclipses and comets are the most rare skies. They too, are tied to rarity in real life. Also, some rarity is based off of the amount of time or the difficulty it took to photograph them.
Our rarity table has 4 layers with a lot of different elements: Landscapes, skies, foregrounds and atmosphere.