3GPP 24.501 v20.0.0 — the document's own text
8.2.30.9 NAS message container
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 if the UE is sending a CONTROL PLANE SERVICE REQUEST message as an initial NAS message and the UE needs to send non-cleartext IEs.