You are not required to enroll as a student if you are completing your doctoral studies at the FAU, but you may decide to do so if your residence permit or scholarship depends on your status as a student, or if you want to take advantage of certain benefits available to FAU students, such as the semester ticket or special rates on your health insurance.
Doctoral students are allowed to enroll as a student of the FAU for six semesters. If you decide to do so, you are obligated to follow the same enrollment requirements as all other students, including paying student fees and complying with re-enrollment deadlines. In addition, you should follow the instructions for students of “open-admission study programmes” (zulassungsfreie Fächer).
More information can be found at the homepage of the Student Administration.