I think it is an assessment based on how badly inflamed and damaged the joints are, degree of deformity of joints, resistance to treatment, progression in spite of treatment. It isn't one thing, or a ahrd and fast set of criteria, it is more your Drs assessment based on his experience and how bad RA can get. Early on, people tend to have milder disease, and if treated early, the joint damage can be prevented to some degree. Everyone's disease is a little different-some more aggressive than others. The presence of complications, such as pulmonary involvement, occurs in more severe disease.