3GPP 24.501 v20.0.0 — the document's own text
8.2.11.4 5GMM cause
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The AMF shall include this IE when the Payload container IE contains an uplink payload which was not forwarded and the Payload container type IE is not set to "Multiple payloads".