Your list of Departmental courses serves two purposes: it defines the physics program you need to graduate and it forms one of the elements used in computing the departmental average for the purpose of class ranking, honors, and prizes. This note describes how the list of Departmentals is determined.
The list of Departmentals submitted to the Registrar shows that you have met the course requirements to graduate with an undergraduate degree in physics. A minimum list, consisting of eight courses, encompasses most, but not all, of the core curriculum that the Physics Department thinks every major should have taken. University rules require at least eight Departmentals. (Note that JP's and the senior thesis are not counted as Departmentals. They are, however, included in the departmental average.)
Your list of Departmentals must include all of the core courses you take in the Physics Department from PHY 208 through the 300-level, as listed in the Undergraduate Announcement - typically PHY 208 (Quantum Mechanics), PHY 301 (Thermal Physics), PHY 304 (E&M), PHY 305 (Quantum Mechanics), PHY 312 (Advanced Lab), as well as the required Complex Analysis (MAT 317, 331, or MAE 306) and one other 300-level math course. The remaining course or courses should be 300-level "cognates:" courses filling out your plan of study for the physics degree. These can be 300- or 400-level physics courses, graduate physics courses, courses from other technical departments such as biology or engineering, or even departments such as history, as long as the course number is greater than or equal to 300 and the course fits into a well-defined program of study. This flexibility is built into the major to accomodate interdisciplinary study that uses physics as a backbone. Some 300- or 400-level courses, notably organic chemistry, do not qualify. The determination of which courses are acceptable is made on a case-by-case basis by the Departmental Representative.
While you are a physics major, there are various forms on which you are asked to declare which of your courses are Departmentals. You should make your best guess, but not worry about it, beyond insuring that you have at least the minimum eight needed to graduate. Most of the designations are required, as indicated in the previous paragraph. Where there is flexibility, the only criterion would be to maximize your departmental average. This exercise in optimization is done for you by the Departmental Representative just before you graduate. After including the required courses and determining which allowable cognates have the highest grades, the DepRep will add other qualifying courses to the list if they raise your average. This service is provided at no extra charge!