3GPP 24.501 v20.0.0 — the document's own text
8.3.1.3 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 in the message when the UE requests to establish a new PDU session with a DN and requests an SSC mode.