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

D.8.6 Unknown and unforeseen IEs in the non-imperative message part

Taught in 5. Finding your way around TS 24.501 (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).