The CDI System is a way to set categories to types of supernatural phenomenon. It is a three part system used by the Ghostbusters when addressing the paranormal they are coming in contact with.
"Sir, what you had there was what we refer to as a Focused, Non-Terminal, Repeating Phantasm, or a Class 5 Full-Roaming Vapor . . . a real nasty one too"
A clear list system was made in Ghostbuster RPG, which is the standard most fans go by. The class system is known mostly as list of seven classes that a paranormal event can be labeled to better understand and deal with.
These are undeveloped forms, insubstantial and difficult to see. Interaction with their environment is limited and enigmatic (i.e., spectral lights, voices and sounds, ectoplasmic vapors, etc.). Simple application of a proton pack beam is normally effective.
Manifestations focused in this time and space, Class II ghosts and up can physically manipulate things in this world. These forms are vague, inconsistent, or incomplete (i.e., floating sheets, ghostly hands, animated lips, etc.). Although a proton pack beam is normally effective, some Class II ghosts have the capacity to return attacks.
Anonymous Hauntings. Distinct human form and personality is evident, but former identity (I.E., as a living being.) is not established. If established, ghost is reassigned as Class IV. Often difficult to deal with, Class III ghosts generally possess sophisticated means of defense.
Identity is established. They have a distinct human form and personality with known identity, such as General Custer or Cleopatra. Economic disposal methods include research into the background of said entity, as well as possible communication with it.
These are Ectoplasmic manifestations of definite but non-human form. Speculation includes the theory that Class V's are formed from emotionally-charged events or as side effects from ritual summoning (Slimer was a by-product of the rituals that a cult held in the Sedgewick Hotel's basement.). These typically require extensive proton pack implementation to eradicate.
Ghosts from lower life forms. A giant penguin was once seen attacking a mugger in Central Park. Later it was discovered that the penguin was a ghost. Unique solutions are often required to handle these entities, including research into habitats, allergies, natural enemies, etc.
Metaspectres. Obsessively malevolent, exceptionally powerful, and exhibiting control over subordinate forms, such entities are potentially very dangerous. These are often identified by primitive cultures as "Demons". Entitles which fit this classification include Gozer and Zuul. Neutralizing them is usually a problematical undertaking at best. Most standard procedures are futile. The most realistic plan is to take measures to prevent these things from entering the sphere of influence in the first place.
- Class 1
Class 1 entities are lower-powered manifestations that lack enough psychokinetic mass to affect the physical environment. As a result, they tend to fade in and out of the physical plane. They are characterized as not being fully developed and as echoes of feeling. These entities are only capable of localized cold spots, unexplained lights and/or sounds, and occasionally, telekinetic contact. 
- Class 2
Class 2 entities have a slightly stronger grip on the physical plane than a Class 1 but also does not fully manifest. They do have the ability to possess inanimate objects for a longer span of continuous time. 
- Class 3
Class 3 entities are humanoid and exhibit general behavior. The types of manifestations vary from a corporeal form virtually indistinguishable from a living person to a mockery of humanity, with distended features and exaggerated behaviors. The constant factor of a Class 3 is their identity is unknown, potentially to themselves as well.  Identifying an entity has varied results. A Class 3 who knows its identity will either pass on to the next plane of existence or more fully resist the urge. 
- Class 4
Class 4 entities are like Class 3's in that they have a human form. However, a Class 4 knows its identity and possesses memories or replicas of memories from their past life. Dealing with a Class 4 can require research into the life of that person in order to solve the case at hand; typically that of their wants desires, addictions, and habits. A Class 4 is capable of immense power if there is a source to draw from, such as the collection of Psychomagnotheric Slime known as the River of Slime or the raw ambient energy of Gozer as it built up towards its next manifestation. In these cases, the Class 4 can exhibit power that is typical of a Class 6 or Class 7. 
- Class 5
Class 5 entities are fully formed manifestations most often the by-product of highly charged emotions and ambient psychokinetic energy. They can have human characteristics like limbs and a discernible face but otherwise has a non-human form and lacks personality and/or intelligence. 
- Class 6
A Class 6 entity manifest in forms and behaviors that are exclusively animal in nature. While they match the appetites and instincts of their living counterpart, a Class 6 can manifest in varying sizes. 
- Class 7
A Class 7 entity is the most powerful class and includes godlike beings, demons, and other powerful manifestations. They are capable of communication with humans and are not totally alien in their thought processes, desires, emotional needs, and their origins. Class 7 entities are to be avoided and usually luck is the only way to survive an encounter. 
Ghostbusters: The Video Game Realistic Versions
- Ghosts: Spirits or souls of the deceased.
- Corporeal: Tangible manifestations.
- Deity: Supreme Beings.
- Unstable: A physical manifestation who forms itself with paranormally compromised items.
- Environmental: Ectoplasm, its Sub-Classes, and effects of.
- Legends: Entities popularized by mythologizes, folklore, and legends.
- Focused: The ghost is tied to the specific locale in which it first appeared and cannot stray from it.
- Free Floating: A ghost that moves about at will, with no regard to buildings, people, or time and space, as we know it. Sometimes just called "free".
- Full Torso Some ghosts have full torsos, some have arms and legs, and some have no head and the like. Full torso apparitions are relatively rare - many hauntings are indistinct below the chest.
- Repeating or Repeater: May refer to a self-regenerating P.K.E. force (Common with ritually-summoned spooks) or to a spook with obsessive repetitious behavior patterns (i.e., Always manifesting at a certain place and time). Even when the ghost can be captured or eliminated, some agency continues to produce successive hauntings. Such ghosts reappear periodically until the source can be disabled.
- Vapor, Vaporous, or Mist: A common but imprecise term, it may refer to a Class I or II entity, or any insubstantial haunting.
Ghostbusters: The Video Game Realistic Versions
- Anchored: Indicates that a ghost is tied to a particular location.
- Animating or Animator: An entity capable of animating an object whether by telekinesis or by inhabiting them.
- Caustic: Corrosive, burning, or just plain dangerous.
- Composite: A collection of things.
- Corporeal: Tangible or having a physical body.
- Ethereal: Disembodied or intangible.
- Floater: An entity capable of limited flight.
- Free-Roaming: An entity that roams freely and is not bound to any location.
- Free-Roaming Vapor: A wispy entity that simply roams around freely.
- FRVP: A free-repeating vaporous phantasm. Sometimes a grotesquely altered human form.
- Full-Torsoed:It is an entity with no lower body.
- Inhabiting: A creature that occupies and binds to an inanimate object, like a possessor, but one that acts out of pure instinct rather than higher reasoning.
- Kinetic: Of or relating to movement.
- Paranormal: Beyond normal; Strange, unnatural, or unknown.
- Plane or Planar: Realm or dimension, or of a realm or dimension.
- Possessor: A ghost capable of possessing objects or people.
- Reactive: Responsive.
- Remnant: A spirit that remains in this world as a result of being unable to completely cross over, as opposed to those that choose to return to the mortal plane.
- Secretion: Something sticky and oozy, mucus or slime.
- Seep: Short for seepage; see Secretion
- Swarmer: An entity that travels in packs and swarms around its prey.
- Telekinetic: Having the ability to move things without physical or ectoplasmic contact; having the ability to move things from a distance.
- Transdimensional: Outside of this dimension.
- Ultradimensional: Across of this dimension.
- Wandering Possessor: A wandering entity that can possess objects or people.
Please go here.
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "This cheat sheet is to help you familiarize yourself with the 7-level classification systems."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "It should be noted that classes are not strictly organized by way of raw psychokinetic power."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "Your basic catch-all, lower-powered manifestation. Lesser voices, entities that don't have enough psychokinetic mass to affect an environment in a substantial way. They're not fully developed, and are generally echoes of feeling. They may cause for localized cold spots, unexplained lights and/or sounds, as well as occasional light telekinetic bump. Their hold on this plane of existence is tenuous at best, and they can fade in and out of our own sphere of realization."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "A Class 2 has a slightly stronger grip on the material world than a Class 1, but not strong enough to manifest as a fully formed presence. Partial formations are enough to affect and manipulate the environment, as well as possess inanimate objects and to do so for a longer span of continuous time."
- ↑ Ray Stantz (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.7). Ray Stantz says: "And that's the real difference between Classes 3 and 4 – knowing who they are. Never forget that identity is a powerful thing, you know – as a tool, as weapon – even moreso in the afterlife than it is otherwise."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "Class 3 manifestations are recognizably humanoid in form and general behavior, but beyond that, the sky's the limit. They can present as corporeal (solid form/tangible) and be virtually indistinguishable from a person -- except for the ability to do things like vanish into thin air, or spin their head around -- or they can look like a mockery of humanity, with distended features and exaggerated behaviors. The constant of class 3 is that, typically, their human identity – if they had one – is unknown, potentially even to them. Figuring that out can result in a no-fuss conclusion to a Class 3 haunting, or it can simply give the entity an identity, which can and often does allow them to more fully resist that call to the next plane of existence."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "Like a class 3, a class 4 manifestation leans towards a regular human shape and personality -- with one major addition, that of an identity. They know who they are (were) and hold the memories (or as I have come to believe, replicas of the memories) that they held in life. Dealing with a Class 4 can become easier with research into the person that they were; wants, desires, addictions, and habits tend to remain consistent. Beyond that, anything is possible. A class 4 entity is capable of immense power; please see our case file on Vigo the Carpathian for a good example -- the short version is that this particular class 4 formed a symbiotic relationship with a cache of psychomagnetheric ectoplasm and gained enough power to rival a class 7."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "Class 5s are fully formed manifestations (as opposed to a floating pair of eyes, for example) that may have human characteristics, but a decidedly non-human form overall. A prime example of this is the green ghost in the lab's observation tank. It has limbs and a discernible face, but otherwise shares little in common with a humanoid. They can exist without personality and discernible intelligence, and are are most often the by-product of highly charged emotions and ambient psychokinetic energy. (In theory, studies are ongoing.)"
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "Like a class 3, a class 6 is recognizable in form and behavior -- but that form and those behaviors are exclusively animal in nature. With appetites and instincts to match their living counterparts, class 6 entities can make for some of the most dangerous regularly appearing manifestations. Especially as they are not necessarily restricted to a particular size and sometimes they figure that out."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #1" (2017) (Comic p.23). Class Notes reads: "The most powerful class, which includes godlike beings, demons, and other powerful manifestations. While they are capable of communication with human beings, they are not totally alien in their thought processes, desires, emotional needs, and yes, more often than not, their origins. This is an entity we do not want to meet, as luck is usually the only way to survive an encounter."