In summary:

Codex Justianius = Roman statutory law (as developed over 7 centuries) which was consolidated under Justinian in 529 CE

Novelle leges = New constitutional law added by Justianian around 533 CE

Instituitones = The 'textbook' for students of Codex written 533 CE

Digesta = Jurist opinion and application of Justinian code ('treatises') compiled around 533 CE, what would be considered 'glossing' in the middle ages

For shits and giggles, the Justinian code was 'recovered' in late 11th early 12th c by 'Gratian' among other Bolognese scholars and jurists and made the foundation of reconciled Canon Law by the Papacy in 12th c. However fragments of the Justinian Code survived through the early middle ages in the various 'barbarian' laws.