I found a part of my research! When I was working on this one story, I put all of what I'd researched into a "register" so I could keep things straight in my head about what fitted in where. This is how it shook out...
Categorical Ability Register
Natural: To have abilities within the boundaries of Nature as they exist on Earth. Normal human beings and animals fit into this category.
Preternatural: To have abilities that extend the boundaries of Nature as they exist on Earth. Beings that were once human or have a human “side” or essence fit into this category, i.e. vampires, werewolves, shapeshifters, certain types of ghosts, slayers, reapers (as helpers of Death), humanoid aliens (Superman, Saiyans), mutants, witches and wizards, fairies/brownies, dwarves, elves, humans or huminoids with extrasensory abilities (psychics, empaths, telekinetics; i.e. Carrie, Jedi Knights and Sith Lords), and the like. This description also includes the offspring of supernatural beings and humans since demigods, half demons, etc., are not true immortals despite possibly having more ability than either parent.
Supernatural: To have abilities that operate outside the boundaries of Nature as they exist on Earth. This includes beings that were never human, but were divinely created directly by God or the Devil, i.e. angels (heavenly, former, and fallen), demons, and certain types of spirits and ghosts (i.e. Angel of Death, elementals). Supernatural beings have the trait of being truly immortal – they cannot be killed. Those that can be killed cannot be killed by a mortal being of any kind (Pre-, Supra- or Natural), though it is possible that another supernatural being may be able to do so, typically the one who created it. Certain gods and goddesses, devis, and the like, fall into this category. This also includes human beings who have died can become immortal, making them supernatural beings, i.e. Hercules, guardian angels or vengeful demons (Spawn), etc.
Supranatural: To have abilities that extend beyond the boundaries of Nature as they exist on Earth. Similar to Preternatural beings except the entity does not have a connection to humanity and/or have a human-like appearance, i.e. space aliens (Predators, Aliens, ET, etc.), beings from other dimensions, etc.
The Cliff Fall Example:
If a Natural being falls of a high cliff, they will die or at least be seriously injured.
If a Preternatural being falls off the same cliff, he/she will survive or at least be far less injured than the Natural being.
A Supernatural being would not fall off the cliff since they have the ability to fly, float, disappear, or land without incident or injury.
A Supranatural being would sustain injury or death according to the structure of its body as created on its own world.
For example, ET would likely be able to float or fly to avoid injury. A Predator would likely sustain some injury but avoid death because of his or her toughness and/or technological advancements.
The Possession Example:
If an alien (Supranatural) possesses a human being (Natural), the result is a Preternatural being since there is/was a human body and essence as the foundation of possession; however, a Supranatural is infusing the human being with a portion or all of its innate qualities during the inhabitation, which gives that human body additional abilities it did not have prior to the possession. Thus, it is now Preternatural.
- Please Note: Possession may also occur without any additional abilities transferred to the human being, which may lend itself to the low innate abilities of the alien or show an unwillingness of the alien to commit its powers to the human body. This occurs for a variety of reasons, with one being that the alien is planning to leave its human host after a short period of possession (i.e., Starman).
Examples: Men in Black. Invasion of the Body Snatchers. K-PAX. The Astronaut’s Wife. The Hidden. Species III.
If a demon (Supernatural) possesses a human being (Natural), the result is a Preternatural being since there is/was a human body and essence as the foundation of possession; however, a Supernatural entity is infusing the human being with a portion or all of its innate qualities during the inhabitation, which gives the human body additional abilities it did not have prior to the possession. Thus, it is now Preternatural.
- Please Note: Possession may also occur without any additional abilities transferred to the human being, which may lend itself to the low innate abilities of the demon or show an unwillingness of the demon to commit its powers to the human body. This occurs for a variety of reasons, with one being that the demon knows its full powers will overwhelm the human host after a short period of possession, forcing it to abandon the body; whereas less power will cause less damage and thus, extend its stay.
Examples: The Exorcist. Ghost Rider. The Shining. Evil Dead. Paranormal Activity.
Typically, a Preternatural cannot possess a Natural since they generally do not have the power or innate ability to do so. There may be instances when the Preternatural may possess a Natural through the use of outside means that enable possession, usually through the use of Supernatural processes. Atypically, a Preternatural such as psychics, mutants and witches, may have the innate ability to temporarily possess a Natural.
A Supernatural may be able to possess other Supernaturals according to innate ability or outside means. The same may be said for Supranaturals.
A Natural cannot possess other Naturals or other Beings within the Register without Super- or Supranatural means. (No, co-dependent relationships don't count)
Hope that helps clarify things for you.