These are the views of the three masters of the school: Abū Ḥanīfa, Abū Yūsuf and Muḥammad. Zufar and al-Ḥasan are sometimes added to them. They are called ẓāhir al-riwāya because they reach us in six books Muḥammad wrote, passed on by trustworthy men. So the books are Muḥammad's, and the views belong to the school. Ibn ʿĀbidīn says these reports come down to us either as mutawātir, mass transmitted, or as mashhūr, widely known. You give fatwā on them even where no scholar has explicitly declared them sound.
For the six books, and the order Muḥammad wrote them in, see Marātib al-Kutub in the next branch. The list used to sit in both places. It now sits there only, because that branch is the one about books.
Al-Shaybānī (d. 189H) also wrote works called Masāʾil al-Nawādir. In those, the chain of transmission is not clearly shown in the books. That is why they are called "not ẓāhir al-riwāya."
First class: other books of Muhammad's own, besides the six; like al-Kaysaniyyat, al-Haruniyyat, al-Jurjaniyyat and al-Raqqiyyat
Second class: books not by Muhammad; like Kitab al-Mujarrad by al-Hasan b. Ziyad (d. 204H), and the Amali of Abu Yusuf (d. 182H)
Third class: solitary narrations; like Ibn Samaʿa (d. 233H)'s narration, Muʿalla b. Mansur (d. 211H)'s, and others on specific issues
What sorts usul from nawadir is which BOOK a view landed in, not who held it. Both levels are the same masters of the school; the six books are usul, and everything of theirs outside the six is nawadir
Verse 16 calls this rank "al-nawāzil." It is a later tier, and it is separate from al-Uṣūl and al-Nawādir.
These are cases that later mujtahids worked out themselves, where they found nothing narrated from the founding imāms.
First came al-Nawāzil of Abū al-Layth al-Samarqandī (d. 373H). Then Majmūʿ al-Nawāzil wa al-Wāqiʿāt of al-Nāṭifī (d. 446H). Then al-Wāqiʿāt of al-Ṣadr al-Shahīd (d. 536H).