3GPP 24.501 v20.0.0 — the document's own text
8.2.6.42 AUN3 indication
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE shall be included in the REGISTRATION REQUEST message when the 5G-RG is acting on behalf of an AUN3 device performs registration procedure for initial registration.