3GPP 24.501 v20.0.0 — the document's own text
7.6.4.1 IEIs unknown in the Type 6 IE container information element
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The UE shall ignore all IEs with an IEI unknown in a Type 6 IE container information element which are not encoded as "comprehension required" (see 3GPP TS 24.007 [11]).
The network shall take the same approach.
NOTE: The set of IEIs defined for a Type 6 IE container information element is independent of the set of IEIs defined for other parts of the message. Therefore, an IE can be unknown in a Type 6 IE container information element, even if an IE with the same IEI is known in other parts of the message, and vice versa.