3GPP 24.501 v20.0.0 — the document's own text
8.3.3.3 Allowed SSC mode
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 when the network rejects the PDU SESSION ESTABLISHMENT REQUEST with cause #68 "not supported SSC mode.