This is a reference to the early legal schools of the Ḥijāz and Iraq. Hossein Modarressi cites this reference to argue that the legal legacy of the Family of the Prophet differed in its sources and approaches from these schools.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).