A habitable planet refers to a celestial body located within the habitable zone of its star system and possessing potential habitability. Due to the biological diversity among species, each species and civilization has its own criteria for habitable planets. The following classification reflects the standards of the human race, as defined by the Okilok Human Federation. The Federation categorizes habitable planets into five primary tiers, ranked from A to E:
In general, Class A planets are exceedingly rare in the universe. All currently known Class A planets have undergone some form of planetary environmental modification.
Furthermore, each major class (A to E) is subdivided into levels from 1 to 10, with higher numbers indicating lower habitability. For instance, Yejiesa—the capital of the Okilok Human Federation—has been rated A9 following extensive environmental engineering. Fluvina II, home to the Fluvinians, is rated D6, meaning that aside from the Fluvinians, most human subgroups cannot survive on the planet’s surface without protective gear or artificial habitat conditioning.
The term "species" is traditionally used to define fundamental biological units. However, following the Gene Editing Frenzy during the era of the Dark Empire, the classification of species has become neither consistent nor strictly scientific.
Among the Four Great Human Powers, all known lifeforms are generally categorized into two broad types: sapient species and non-sapient species. Sapient species are further divided into humans, sub-hominid human variants (classified as Homininae hominoids, hereafter referred to as subhumans), and others.
In the brief span of human interstellar history, humanity has encountered several thousand sapient species—all of which are carbon-based lifeforms, including mammalian, reptilian, fungal, and other biological categories.
Homininae hominoids—hereafter referred to as subhumans—are a subspecies group within the broader human clade. Genetically, their similarity to standard humans ranges from 66% to 99%. Subhumans cannot be reliably identified by appearance alone. Due to the vast environmental diversity across planetary systems (with over 20 B-class habitable worlds, more than 80 C-class, and countless D- and E-class worlds in human-controlled space), even genetically identical individuals may develop vastly different physical traits depending on their environment of birth and evolution.
Thus, the first and foremost rule of survival in human society is: never judge a person by appearance alone.
As time progresses, the taxonomy of human species has grown increasingly complex. Nevertheless, several basic classification protocols remain widely accepted across human space, particularly regarding the distinction between humans and subhumans. According to these standards, a being is defined as human if it meets two criteria:
Visual conformity to human norms, as established by archival records from the early Age of Interstellar Navigation (based on standard Earth human features);
Genetic similarity exceeding 97% to the Earth human genome, also based on those archival references.