I would say that diagnostics are clues that in the case of the human body that indicate a medical condition. For example, someone might have a heart mitral valve major leak and feel severe shortness of breath, and find it very difficult to walk, especially uphill . Or in the case of developmental disabilities, a person with autism spectrum disorder may need instant gratification and also not think of the consequences of his/her actions and this would form part of the diagnosis of their condition.