The context is often important. The matter is very complex, and often uncertain. That is not the case. I will try to explain some key concepts. Nosology is the study of the classification of disease.
Epistemology is the branch of philosophy concerned with knowledge.
If you don't know the difference, you are probably a realist. I am a nominalist. The distinction between a nominalist and a realist is not that a nominalist doesn't believe that there is some phenomenon that is being named. It is that the name of a thing is not the same as the thing that is being named. It is essentially that there is a difference between the map and the territory.