3GPP 24.501 v20.0.0 — the document's own text
8.3.14.8 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 trigger the re-establishment of the PDU Session with the alternative S-NSSAI in SSC mode 2 or SSC mode 1.