3GPP 24.501 v20.0.0 — the document's own text

7.7.1 Syntactically incorrect optional IEs

Taught in 7. What a NAS message looks like on the wire (CT1 — the group that writes what your phone says, in depth), 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).