These are underdeveloped forms.  Lower-powered manifestations that lack enough psychokinetic mass to affect the physical environment, Insubstantial and difficult to see without the assistance of Para-Scopes (a.k.a. Ecto-Goggles). Whatever environmental interaction they ARE capable of is at best limited, and enigmatic (e.g. Spectral Lights, voices and sounds, ectoplasmic vapors, levitating objects, etc.) and as a result, they tend to fade in and out of the physical plane. Although not normally necessary, and in some cases considered overkill, these manifestations are usually easily dispatched with a Neutrona Stream or a P.K.E. zap. It can also be a byproduct from a higher class of entity, usually (but not limited to) an ectoplasmic materialization including the viscous, mucus-like slime, but can also manifest as food, water, icicles, and other simple physical forms of varying degree. 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. This includes small volumes of the Psychomagnatheric Slime (Mood Slime for short) that manifested from the Negative energy from the 6 million completely miserable New Yorkers during the Vigo case of 1989. In such small volumes, the Slime is inert, and doesn't carry as much supernatural potential.

Generally, contact with ectoplasm is no more harmful, detrimental, or annoying than getting splashed with water, unless there's other external factors in play (such as the charge of energy that the Mood Slime holds affecting the emotional state of whatever it touches after the fact.), However more research needs to be conducted, and it is recommended either way to avoid contact when possible.

Class 3 manifestations are typically anonymous hauntings. No names, and no distinguishability other than being a human-shaped form, and in many cases, a usually visually blatant profession or hobby, such as a Chef, or a Jogger. Class 3 entities are usually 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. 

If an identity (such as with a name) is able to be established, then the entity is reassigned to a Class 4. As some past civilizations believe that knowing the name of an entity will grant a limited power over it, Many believe that 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 3 manifestations, while still prone to the P.K.E Taser, and Neutrona Wand, are a bit more difficult to deal with, as they often possess sophisticated means of defense.

Speaking of possessing, due to the larger pool of P.K.E. to expend, Class 3 manifestations are able to possess, manipulate and control larger objects, such as statues, furniture, vehicles, and yes.....even People.

However, sometimes it will require a specific kind of emotional energy (whether it's the kind that's held by mood slime, or the kind an entity hungers for in it's host) for a subject to be able to be controlled. And knowing that, sometimes a drastic, jarring change of vibe/energy is the best way to extract a spirit from a person or object (and the best and safest way to do that would be a rapid and continued application of opposite polarity Mood Slime, as seen with Janosz Poha and Dr. Ray Stantz.)

Class 5 manifestations are defintely a completed and blatantly non-human form. In fact, more often than not, they're a manifestation from emotionally charged, or traumatic events, or as a by-product of a ritualistic summoning. They tend to be 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. A prime example of this is the Ghost called "Slimer" being a by-product of rituals held in the basement of the Sedgewick Hotel. They will usually require a liberal dose of Proton Streams to entrap or disperse.

A Class 7 entity is the most powerful class (which includes godlike beings, demons, metaspectres), the manifestations of which are obsessively malevolent, exceptionally powerful, and can exhibit control over lesser, subordinate forms, making such entities a VERY dangerous sort. Often identified in primitive cultures as "demons", This would also come to include "Gods", "Demigods", they are capable of communication with humans and are not totally alien in their thought processes, desires, emotional needs, and their origins.

Examples that come to mind that fit the descriptors would be Vinz Clortho, Zuul, and Gozer, and Garraka. 

MOST of the time, Neutralizing them is a problematic undertaking at best-as most of our Standard Procedures are futile as they are attuned to the frequency of ectoplasm, and Many high powered manifestations don't have it. The Most practical strategy in dealing with such a threat is to prevent these otherworldly beings from manifesting into this plane of existence in the first place. Otherwise, Class 7 entities are to be avoided as usually luck is the only way to survive an encounter, and even then, it's a slim chance.