The Codex Justinianus (henceforth CJ for convenience) is one of the volumes of what, by the High Middle Ages, people call the Corpus Iuris Civilis, along with the other juristic/juridical/legal works of Justinian, the Digest (or Pandects), the Institutes, and Justinian’s own Novellae Constitutiones — these being the new constitutions that post-date the other work. CJ is itself an anthology of excerpts from imperial laws arranged thematically; some laws thus get themselves included multiple times. They date from Hadrian (r. 117-138) to Justinian (r. 527-565).
The Digest is the opinions of jurists where the laws conflict, a reality made manifest by CJ. It is mostly Ulpian (c. 170-223) and Paulus (2nd/3rd c. AD) The Institutes are the work of the Roman jurists, largely Gaius (108-178), mostly from the High Imperial period. These are texts that discuss how to apply the law in different cases.
The Corpus Iuris Civilis demonstrates to us the fact that the eastern Roman Empire, based in Constantinople (but poised to [re]conquer Africa and Italy), did not simply imagine itself to be the successor to ancient Rome but, in a very real way, was. Justinian’s consuls stand in a direct succession that saw itself receding back to Brutus in 509 BC and the foundation of the Republic. And Justinian’s desire to consolidate and clarify law, something attempted in the third century (Codex Gregorianus and Codex Hermogenianus) and fifth century (Codex Theodosianus), but not with as much staying power as Justinian’s work (esp. not the former two).
Thus, in CJ, Justinian does not restrict himself with the world after Constantine, as Codex Theodosianus had. He does not think only in terms of life in Constantinople. He sees that Roman law, taught in Beirut and applied in Constantinople, Antioch, Alexandria, is a living body of laws that reach back to the days when an emperor resided in the Eternal City herself.
Because of this vision, I would argue that Justinian’s Novellae, despite some novelties that arise because of shifting circumstances, are themselves a natural outflow from the living tradition of Roman law. I will write more about living tradition someday soon, but it is an important idea to keep in mind when we look at the Roman and mediaeval (and all pre-modern) worlds. The process, content, and conceptualisation of the Novellae exist alongside the rest of the Corpus, alongside centuries-old laws in CJ, resulting in something that somehow is an outworking of that older tradition.
Late Antiquity is still antiquity, and Justinian, even as he forges a brave, new (‘Byzantine’) world, is part of antiquity. The world is shifting and transforming, yes — but it always has. Hadrian’s world is not Augustus’, Alexander Severus’ is not Hadrian’s, Diocletian’s is not the Severans’, Theodosius I’s is not Constantine’s or Diocletian’s, Justinian’s is Theodosius I’s — but they are all linked together by various traditions of the Roman world, including law.