This is a reference to Jaʿfar al-Ṣādiq's position on recognizing the validity of the rules of other religions, such as marriage among the Magians, for example. Hossein Modarressi cites this reference as an example of one of the legal presumptions of al-Ṣādiq's legal school: the rule of recognition.
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).