Mild, Moderate-Severe and Fulminant-Severe categories are largely based on response to intervention these days. So if your Crohn's is under good control with minimal meds (ie Pentasa) with or without surgery you are considered Mild at this time. If your Crohn's requires immune modulators to attain remission then you're Moderate. If your Crohn's requires biologics to attain remission then you're Moderate - Severe (even if your symptoms weren't all that bad but your gut was a mess). Fulminant-Severe is pretty much reserved for those folks who do not achieve clinical remission from meds or surgery - failed remicade and/or humira along with lesser drugs and usually have wide ranging severe symptoms and may have multiple AI disorders (lupus, diabetes, RA, AS, etc) suggesting a broader underlying disorder of the immune system.
It's confusing because your classification can change very quickly from Fulminant-Severe to Mild if surgery is required and it is followed by remission. All the categories are blurry and your doctor may or may not agree with this definition. Our experience is that doctors tend to categorize you in the best possible group as they do not like to be the bearers of bad news any more than anyone else does. IMHO they will tell you Mild or Moderate but avoid Severe if at all possible. Fulminant-Severe is apparently a category of ZERO though all of us know folks who are in that group, sadly enough.