3GPP 24.501 v20.0.0 — the document's own text
8.2.7.7 Configured NSSAI
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The network may include this IE if the network needs to provide the UE with a new configured NSSAI for the current PLMN or SNPN and the UE is neither registering nor registered for onboarding services in SNPN.