3GPP 24.501 v20.0.0 — the document's own text
8.2.3.3 AUN3 device security key
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 when the AUTHENTICATION RESULT message is provided to a 5G-RG that is acting on behalf of an AUN3 device if the EAP message IE is set to an EAP-success message.