They really weren't very consistent, no. In the cartoons, they pretty much just went with a "power scale" type system, as opposed to
the system that as far as we know was first seen in the Ghostbusters RPG/Ghostbusters International by West End Games, which seemed to be based more on the entity's origin and level of impingement into the material plane.
Like a lot of writers--and fans--I like the GBI system better.
As I put it
once before:
--Remember that it's not a power scale. The Class VI seems to be the "odd one out" (but more on that in a moment) but remember, Slimer would be a Class V, Vigo would be a Class IV (I'm just not sure he's quite enough to be upgraded to Class VII, though if he'd taken over the world he sure would be). Vigo is a lot more powerful than Slimer.
You can actually break down the first six classes into groupings of two:
--Class I and II are indistinct/incomplete. Their prior nature can be difficult to ascertain. You can almost think of them as fractions of Class III-VI ghosts.
--Class III and IV are ghosts of humans. The only real difference is that you know who a Class IV used to be. I admit this seems a bit odd to base the difference on, but knowing a ghost's pre-death identity means you have access to more information about it, and thus clues on how to deal with it. Again, the power level is meaningless here--it's easy to imagine a Class III being more powerful than many Class IVs.
--Class V and VI are non-human. V is unidentified, possibly totally extradimensional. VI is the ghost of an identified non-human life form. In some ways, you could say the real "difference" is the same as III and IV--you know what a Class VI used to be, while with V you do not.
--VII of course is the "super" class, Metaspectres, powerful entities usually of extradimensional origin. In a way, it's a catch-all--Class VIIs could have traits similar to a Class III-VI entity, but in this case it IS a power assessment. An inhuman entity who eats everything and slimes you is a Class V. An inhuman entity that can sneeze and turn Cleveland into charcoal is a Class VII.