3GPP 24.501 v20.0.0 — the document's own text
8.3.9.18 Alternative S-NSSAI
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE shall be included when the network needs to provide the alternative S-NSSAI to replace the S-NSSAI of the SSC mode 1 or SSC mode 3 PDU session.