What is the difference between classification and diagnosis? i often see classification written in studies rather than diagnosis.

Depends: Classification may include several conditions under a group of disease condition or the severity of a medical condition. A diagnosis is the name of a specific medication condition.

Depends on condition: Diabetes can be classified as type 1 or 2 depending on a variety of parameters. Cancer can be classified into stages based on how far the cancer has spread. Without the specifics of the condition in question it is difficult to say more than it is a matter of words. Diagnosis is a condition. Classification puts that condition into categories that are useful to those who treat them.