Under the millet system, non-Muslim people were considered subjects of the Empire, but were not subject to the Muslim faith or Muslim law. The Orthodox millet, for instance, was still officially legally subject to Justinian's Code, which had been in effect in the Byzantine Empire for 900 years. Also, as the largest group of non-Muslim subjects (or zimmi) of the Islamic Ottoman state, the Orthodox millet was granted a number of special privileges in the fields of politics and commerce, and had to pay higher taxes than Muslim subjects.
What code had been active for 900 years in the Byzantine empire?
Justinian's Code