3GPP 24.501 v20.0.0 — the document's own text
8.2.10.11 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).
The UE shall include this IE if the UE can provide alternative S-NSSAI associated with the S-NSSAI to be replaced to the network.