Types of Cancer typically seen in a hematology/oncology department would be leukemia, Lymphomas, Hodgkin, non-Hodgkin, multiple myelomas and immunological disorders. A hematologist oncologist is a hematologist that specializes in the diseases mentioned above. A hematologist oncologist cannot cure operable cancers like prostate cancer.  

Typically, oncology is a department which can observe cancers that have need of surgical cure like ovarian cancers, thyroid cancer, digestive and throat system cancers etc. Patients would be seen by an oncologist or a surgeon with experience in cancer surgery. Some doctors have received special training, or have experience with a certain kind of medicine, and may look after and heal patients by their selves with no reference with them to an expert.  

Because disorders and diseases seen in hematology/oncology often overlap, it is more effective to have hematologists working closely with oncologists. Many patients see both hematology and oncology doctors during the course of their therapy. A breast cancer patient, for example, may be treated with a bone marrow transplant or stem cell transplant, and her oncologist would work in conjunction with the hematologist. Another benefit to doctors and patients is the hematology/oncology clinic is equipped with special microscopes and often has their own lab, which enables the doctors and medical technologists to make rapid diagnosis, or observe patients efficiently and quickly. Oncologists and Hematologists are trained very well for the specialization and they have the ability to recognize cells under the microscope that general-practice doctors typically do not have.   You are correct in stating that not all blood disorders are cancer. Most anemias, like iron deficiency anemia and infectious agents. Anemia caused by red cell defects are surely not cancer containing but they are healed by hematologists.

Again, the study of hematologists is not ALL about the forms of blood disorders, non-cancerous or cancerous and they are not oncologists.   Mostly large hospitals regularly own a hematology lab. The blood test is taken of inpatients mostly there. The CBC (Complete Blood Count) is the most well known hematology test, and is run on most in-patients at some time during their stay. If a blood disorder, leukemia or lymphoma is suspected or founded out, the patient's doctor will guide them to a hematologist, if they feel it is essential.