3GPP 23.501 v20.2.0 — the document's own text
8.1 General
Taught in 8. What actually travels on the wire (The 5G system architecture, in depth).
Clause 8 specifies the overall protocol stacks between 5GS entities, e.g. between the UE and the 5GC Network Functions, between the 5G-AN and the 5GC Network Functions, or between the 5GC Network Functions.