One way to assess severity is the extent to which you act on your delusions or hallucinations (or other symptoms) in harmful (such as criminal or self-destructive) ways.
Another way to classify severity is based on the prevalence and intensity of your symptoms, but that is of course very subjective.
As an example, my by far most intense manic episode may be classified as merely hypomanic because I didn’t do anything destructive and it ended happily (aside from it ending with a descent back into depression). Likewise, several of my less intense episodes can be classified as fully manic because I did impulsive and destructive things and ended up in hospital.
-Albert.