The Humable Range is a scale applied to habitable planets. It takes into account various aspects of the planets ecology and the fragility of its ecosystem to determine the colonization risk.
Despite out best attempts, human introduction into an environment is always destructive to some degree. It is expected that during the first five decades of colonization of a new world that a number of native species will be lost. As well a number of humans. This is called Colonization Loss and is simply an accepted part of the process, but every attempt is made to keep it to a minimum. Younger ecosystems may be unable to adapt to a new outside species and would eventually collapse over decades. as well many worlds may be chemically and environmentally perfect for humans but their ecology might not be. In order for humans to thrive we must be at the top of the food chain. Planets with high ratios of large predators among the wildlife aren’t suitable for colonization. Ultimately we’d end up killing off the top level predators that were a threat to us and thus irreparably upsetting the ecosystem.
When a new world is found probes take samples and thousands of factors are considered. The outcome of these tests determine a planets Humable range. It’s a very narrow window. Planets in the range of 1 to 3 are too delicate for successful colonization, with planets at the low end being even too sensitive to risk any human contact. Planets that fall in a Humable range of 7 to 10 are far to volatile or ecologically hostile for humans to survive. A perfect planet will be a 5, with room on either side. A 5 will be a planet with an established and robust ecosystem that can handle some loss due to the introduction of a new species. Also one where the plant life is mostly non-predatory, meaning everything that grows isn’t poisonous. Most importantly a planet where the top level predators are not big enough to eat people like snack poppers.