3GPP 24.501 v20.0.0 — the document's own text
8.3.7.3 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 to indicate the reason for the deletion of one or more non-default QoS rules, QoS flow descriptions or mapped EPS bearer contexts.