3GPP 24.501 v20.0.0 — the document's own text
8.3.15.2 5GSM cause
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE is included in the message when the UE needs to indicate to the network that an error encountered with a mandatory information element in the PDU SESSION RELEASE COMMAND message.